If I were to give syn oil a try, what brands and weights are preferred for engine, trans (4 speed manual), diff?

Remember your manual transmission likes ATF---I use Dexron III---the rear axle likes SAE 90 Hypoid oil---I mention this because I once purchased an MB with manual transmission that had heavy gear oil rather than the ATF, causing hard shifting. Looked like the kind of thing a "quick lube" shop would do.

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K. Weimer
300SD (1)
300D (5) [Plus 1 parts]
300SEL 4.5 (2)
280SE (4)
280 (2)
250 (1)
250SE (1)
240D (7) [Plus 1 parts]
220D (11) [Plus 3 parts]
200D (2) [Plus 1 parts]
180c (with sunroof)

1995 Nissan UD1800 rollback
"If I can't fix it, it don't get fixed"

There are some Dodge manual transmissions that will actually start a fire if you use regular old hypoid gear oil.... I think they are called "new process" ones.. That would be a shock.. thinking you were doing a good job of maintenance but starting a fire instead....