I don't know the car's history, but I do know it was dealer maintained. The car is tight; no oil leaks anywhere(knock on wood). I'm going to give it a try with the synthetic. First I have to find it. Napa, Autozone, and Pep boys don't carry it. Around me, that is.

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide