If synthetic or mineral oil is used the oil still needs to be changed - and at the right time! Determining the "right time" can be difficult. For mineral oil 2500-3000 miles seems to offer the best protection. For synthetics, longer drains are often possible. There is NO evidence that changing synthetic oil less frequently than mineral oil is bad. The fact that my chain when 94Kmi without stretching is just more evidence this is true. For 603 engines and other engines with hydraulic lifters, there is evidence that synthetic oil will quieten noisy lifters. I determined that SOOT is the most critical contaminant in diesel oil - at least for my purposes. Accepted limit is 3% but I like to keep mine closer to 2%. That translates into 6500-9500 mile intervals for my car and that's how often I change the oil. I go this route because at this rate I change the oil every 3-7 weeks instead of every 2500 miles (1.5-4 weeks) with mineral.

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Brian Toscano