I just switched over to fully synthetic oil in my diesel I have nearly 287K miles on the original engine, and its entire life has been run on the regular stuff. After about 1K miles, the oil finally turned black, and I plan to change it at 4K miles. No leaks anywhere. I love it.

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)