CLAIMS : 1. A method of operating a reciprocating piston internal combustion engine, more particularly a large two-stroke diesel engine, whose cylinders {1) are supplied with fuel, supplied with fresh lubrication oil and drained from used lubrication oil characterized by the step of - measuring a parameter of the fuel or of the used lubrication oil that is indicative of the rate of 3-part wear of engine parts. 2. A method according to claim 1, characterized in that the parameter is the content of an element in the used lubrication oil that arrives in said lubrication oil due to mainly 3-part abrasive wear of engine parts. 3. A method according to claim 1, characterized in that the parameter is the content of the substance involved in the third-part abrasion in the used lubrication oil. 4. A method according to claim 1, characterized in that the parameter is the content of the substance involved in the third-part abrasion in the fuel. 5. A method according to any of claims 1 to 4, characterized by comprising the step of - varying the presence of the substance involved in the third-part abrasion as a direct or indirect function of the content of said at least one element in the used lubrication oil. 6. A method according to any of claims 1 or 5, characterized by comprising the step of varying operating conditions of the engine as a direct or indirect function of the measured parameter. 7. A method according to any of claims 1 to 6, characterized by comprising the step of triggering an alarm to the operator and/or taking measures to reduce the presence of said substance involved in the third-part abrasion when the measured parameter rises above a given threshold. 8. A method according to any of claims 1 to 7, characterized in that the presence of said substance involved in the third-part abrasion is varied in the opposite direction of the change in the measured parameter. 9. A method according to any of claims 1 to 8, characterized in that the presence of said substance involved in the third-part abrasion is set back to normal level when the content of said measured parameter drops below the given threshold within a given time from exceeding the threshold. 10. A method according to any of claims 1 to 7, characterized in that said parameter is measured either continuously or periodically, preferably individually for each cylinder (1) of the engine. 11. A method according to any of claims 1 to 10, characterized in that said alarm to the operator is turned off when the content of said measured parameter drops under a given threshold within a given time from the time of triggering of the alarm. 12. A method according to any of claims 1 to 11, characterized in that a second stage alarm is triggered, if the content of the measured parameter does not drop below said threshold within a given time from reducing the presence of said substance involved in the third-part abrasion. 13. A method according to any of claims 1 to 12, characterized in that said element measured in the used lubrication oil can be any of Cr, Ni, Mo, Ti, W, Mn, and/or Co. 14. A method according to any of claims 1 to 13, characterized in that said substance involved in the third-part abrasion comprises catalyst fines that arrive in the cylinders (1) as a content of the supplied fuel, preferably aluminum silicate. 15. A method according to any of claims 1 to 14, characterized in that the presence of said catalyst fines is reduced by reducing the amount of catalyst fines in the fuel supplied to the cylinders (1). 16. A method according to any of claims 1 to 14, characterized in that the amount of catalyst fines in the fuel supplied to the cylinders is reduced by a fuel treatment device, preferably a fuel treatment system comprising a centrifugal catalyst fines removal stage and/or one or more fine filtering stages. 17. A method according to any of claims 1 to 16, characterized in that said element is preferably Cr and said engine parts comprise preferably hard plated ring grooves and/or piston rings. 18. A reciprocating piston internal combustion engine, more particularly a large two-stroke diesel engine, for carrying out the method according to any one of the preceding claims, with at least one cylinder (1), with which are associated a fuel supply and treatment device (3), a lubricant supply device (7) comprising a supply line (9,9a) for supplying fresh lubrication oil and an extraction line (10,10a) for draining used lubrication oil, characterized by means for measuring a parameter of the fuel or of the used lubrication oil that is indicative of the rate of 3-part wear of engine parts due to catalyst fines in the lubrication oil, and means (12,17,6) for varying the amount of catalyst fines in said fuel as a direct or indirect function of said measured parameter. 19. A reciprocating piston internal combustion engine according to claim 18, characterized in that said parameter is the content of catalyst fines in the used lubrication oil. 20. A reciprocating piston internal combustion engine according to claim 18, characterized in that said parameter is the content of at element that arrives in the lubrication oil due to 3-part abrasive wear. 21. A reciprocating piston internal combustion engine according to claim 18, characterized in that said parameter is the content of catalyst fines in the fuel. 22. A reciprocating piston internal combustion engine according to claim any of claims 18 to 21, characterized by comprising means (12) for varying operating conditions of the engine as a direct or indirect function of the measured parameter. 23. A reciprocating piston internal combustion engine according to any of claims 18 to 22, characterized by comprising means (12,25,26) for triggering an alarm to the operator if the measured parameter rises above a given threshold. 24. A reciprocating piston internal combustion engine according any of claims 18 to 23 characterized by comprising a sensor (22) in said extraction line (10a) for each cylinder (1), or in a collective extraction line (10) for all cylinders (1), for measuring the parameter in the used lubrication oil. 25. A reciprocating piston internal combustion engine according to claim 24, characterized in that the output signal of said sensor (22) is supplied to a control unit (12) that controls the operation of the fuel treatment device (3) and is capable of triggering alarms for the operator. 26. A reciprocating piston internal combustion engine according to any of claims 18 to 25, characterized in that said at least one element in the used lubrication oil can be any of Cr, Ni, Mo, Ti, W, Mn, and/or Co. 27. A reciprocating piston internal combustion engine according to any of claims 18 to 26, characterized in that said element is preferably Cr and said engine parts comprise preferably hard plated ring grooves and/or piston rings. 28. A reciprocating piston internal combustion engine according to any of claims 18 to 27, characterized in that said fuel supply and treatment device (3) comprises means for removing catalyst fines from the fuel, preferably in the form of a centrifugal catalyst fines removal stage comprising a centrifuge (17) and/or one or more fine filtering stages. 29. A reciprocating piston internal combustion engine according to any of claims 18 to 28, characterized in that said fuel supply and treatment device comprises at least two fuel tanks (4a, 4b) for containing fuel with different catalyst fine contents and a valve means (6) to select desired fuel quality. 30. A reciprocating piston internal combustion engine according to any of claims 18 to 29, characterized in that said means for varying the amount of catalyst fines in said fuel comprises a programable control unit, such as a computer (12) with inputs for signals from the sensors (22) and outputs for the centrifuge (17) , the fine filters, the valve means (6) and/or an alarm device (25) . 31. A method for improving the construction of a reciprocating piston internal combustion engine, more particularly a large two-stroke diesel engine, whose cylinders (1) are supplied with fuel by a fuel delivery device (3) , supplied with fresh lubrication oil by a lubrication supply device (7) and drained from used lubrication oil, characterized by comprising the steps of test running the engine, determining the level of 3-part abrasive wear of engine parts, and - when said abrasive wear is higher than a given tolerated degree of wear - changing the engine construction so that less fuel or fuel rests come in contact with the cylinder walls, and/or increasing the capacity of the fuel delivery device (3) to remove the substance from the fuel that causes the 3-part abrasion, and/or improving the wear resistance of the engine parts. 32. A method according to claim 31, characterized in that said level of 3-part abrasive wear is determined by analyzing the drained lubrication oil for the content of at least one element in the lubrication oil that arrives in said drained lubrication oil due to mainly 3-part abrasive wear of the preferably hard plated engine parts. 33. A method according to claim 31, characterized in that said level of 3-part abrasive wear is determined by measuring the engine parts, preferably by measuring the engine part or by measuring the thickness of the hard plating on the engine part. 34. A method according to any of claims 31 to 33, characterized in that the engine construction is changed by changing the type, amount and position of the fuel injectors (2). 35. A method according to any of claim 32 to 34, characterized in that the engine construction is changed by changing gas flow pattern in the cylinders (1) during scavenging and during combustion, in particular the intensity of the induced swirl. 36. An engine with an improved construction as obtained by the method of any of claims 31 to 35.CLAIMS: 1. A method of operating a reciprocating piston internal combustion engine, more particularly a large two-stroke diesel engine, whose cylinders {1) are supplied with fuel, supplied with fresh lubrication oil and drained from used lubrication oil characterized by the step or - measuring a parameter of the fuel or of the used lubrication oil which is indicative of the rate of 3-part wear of engine parts. A method according to claim 1, characterized in that the parameter is the content of an element in the used lubrication oil which arises in said lubrication oil due to mainly 3-part abrasive wear of engine parts. A method according to claim 1, characterized in that the parameter is the content of the substance involved in the third-party abrasion in the used lubrication oil. A method according to claim 1, characterized in that the parameter is the content of the substance involved in the third-party abrasion in the fuel. A method according to any of claims 1 to 4, characterized by comprising the step of - varying the presence of the substance involved in the third-party abrasion as a direct or indirect function of the content of said at least one element in the used lubrication oil. A method according to any of claims 1 or 5, characterized by comprising the step of varying operating conditions of the engine as a direct or indirect function of the measured parameter. A method according to any of claims 1 to 6, characterized by comprising the step of triggering an alarm to the operator and / or taking measures to reduce the presence of said substance involved in the third-party abrasion when the measured parameter rises above at a given threshold. A method according to any of claims 1 to 7, characterized in that the presence of said substance involved in the third-party abrasion varies in the opposite direction of the change in the measured parameter. A method according to any of claims 1 to 8, characterized in that the presence of said substance involved in the third-party abrasion is set back to normal when the content of said measured parameter drops below the given threshold within a given time. from exceeding the threshold. A method according to any of claims 1 to 7, characterized in that said parameter is measured either continuously or periodically, preferably individually for each cylinder (1) of the engine. A method according to any of claims 1 to 10, characterized in that said alarm to the operator is turned off when the content of said measured parameter drops below a given threshold within a given time from the time of triggering of the alarm. A method according to any of claims 1 to 11, characterized in that a second stage alarm is triggered if the content of the measured parameter does not drop below said threshold within a given time from reducing the presence of said substance involved in the third-party abrasion. A method according to any of claims 1 to 12, characterized in that said element measured in the used lubrication oil can be any of Cr, Ni, Mo, Ti, W, Mn, and / or Co. A method according to any of claims 1 to 13, characterized in that said substance involved in the third-part abrasion comprises catalyst fines that arrive in the cylinders (1) as a content of the supplied fuel, preferably aluminum silicate. A method according to any of claims 1 to 14, characterized in that the presence of said catalyst fines is reduced by reducing the amount of catalyst fines in the fuel supplied to the cylinders (1). A method according to any of claims 1 to 14, characterized in that the amount of catalyst fines in the fuel supplied to the cylinders is reduced by a fuel treatment device, preferably a fuel treatment system comprising a centrifugal catalyst fines removal stage and / or one or more fine filtering stages. A method according to any of claims 1 to 16, characterized in that said element is preferably Cr and said engine parts comprise preferably hard plated ring grooves and / or piston rings. 18. A reciprocating piston internal combustion engine, more particularly a large two-stroke diesel engine, for carrying out the method according to any one of the preceding claims, with at least one cylinder (1), with which a fuel supply is associated and treatment device (3), a lubricant supply device (7) comprising a supply line (9,9a) for supplying fresh lubrication oil and an extraction line (10,10a) for draining used lubrication oil, characterized by means for measuring a parameter of the fuel or of the used lubrication oil which is indicative of the rate of 3-part wear of engine parts due to catalyst fines in the lubrication oil, and means (12,17,6) for varying the amount of catalyst fines in said fuel as a direct or indirect function of said measured parameter. A reciprocating piston internal combustion engine according to claim 18, characterized in that said parameter is the content of catalyst fines in the used lubrication oil. A reciprocating piston internal combustion engine according to claim 18, characterized in that said parameter is the content of an element arriving in the lubrication oil due to 3-part abrasive wear. A reciprocating piston internal combustion engine according to claim 18, characterized in that said parameter is the content of catalyst fines in the fuel. A reciprocating piston internal combustion engine according to any of claims 18 to 21, characterized by comprising means (12) for varying operating conditions of the engine as a direct or indirect function of the measured parameter. A reciprocating piston internal combustion engine according to any of claims 18 to 22, characterized by comprising means (12,25,26) for triggering an alarm to the operator if the measured parameter rises above a given threshold. A reciprocating piston internal combustion engine according to any of claims 18 to 23 characterized by comprising a sensor (22) in said extraction line (10a) for each cylinder (1), or in a collective extraction line (10) for all cylinders ( 1), for measuring the parameter in the used lubrication oil. A reciprocating piston internal combustion engine according to claim 24, characterized in that the output signal of said sensor (22) is supplied to a control unit (12) that controls the operation of the fuel treatment device (3) and is capable of triggering alarms for the operator. A reciprocating piston internal combustion engine according to any of claims 18 to 25, characterized in that said at least one element in the used lubrication oil can be any of Cr, Ni, Mo, Ti, W, Mn, and / or Co. . A reciprocating piston internal combustion engine according to any of claims 18 to 26, characterized in that said element is preferably Cr and said engine parts comprise preferably hard plated ring grooves and / or piston rings. A reciprocating piston internal combustion engine according to any of claims 18 to 27, characterized in that said fuel supply and treatment device (3) comprises means for removing catalyst fines from the fuel, preferably in the form of a centrifugal catalyst fines removal stage comprising a centrifuge (17) and / or one or more fine filtering stages. A reciprocating piston internal combustion engine according to any of claims 18 to 28, characterized in that said fuel supply and treatment device comprises at least two fuel tanks (4a, 4b) for containing fuel with different catalyst fine contents and a valve means ( 6) to select desired fuel quality. A reciprocating piston internal combustion engine according to any of claims 18 to 29, characterized in that said means for varying the amount of catalyst fines in said fuel comprises a programmable control unit, such as a computer (12) with inputs for signals from the sensors (22) and outputs for the centrifuge (17), the fine filters, the valve means (6) and / or an alarm device (25). 31. A method for improving the construction of a reciprocating piston internal combustion engine, more particularly a large two-stroke diesel engine, whose cylinders (1) are supplied with fuel by a fuel delivery device (3), supplied with fresh lubrication oil by a lubrication supply device (7) and drained from used lubrication oil, characterized by comprising the steps of test running the engine, determining the level of 3-part abrasive wear of engine parts, and - when said abrasive wear is higher than a given tolerated degree of wear - changing the engine construction so that less fuel or fuel residues come into contact with the cylinder walls, and / or increasing the capacity of the fuel delivery device (3) to remove the substance from the fuel causing the 3-part abrasion, and / or improving the wear resistance of the engine parts. 32. A method according to claim 31, characterized in that said level of 3-part abrasive wear is determined by analyzing the drained lubrication oil for the content of at least one element in the lubrication oil which arises in said drained lubrication oil due mainly to 3-part abrasive wear of the preferably hard plated engine parts. 33. A method according to claim 31, characterized in that said level of 3-part abrasive wear is determined by measuring the engine part, preferably by measuring the engine part or by measuring the thickness of the hard plating on the engine part. 34. A method according to any of claims 31 to 33, characterized in that the engine construction is changed by changing the type, amount and position of the fuel injectors (2). 35. A method according to any of claims 32 to 34, characterized in that the engine construction is changed by changing gas flow pattern in the cylinders (1) during scavenging and during combustion, in particular the intensity of the induced swirl. 36. An engine with an improved construction as obtained by the method of any of claims 31 to 35.