Change the oil weights based on season.. Synthetic or not just stick with the correct weights for temperature expected. Follow the book.

With that said, you sure you want to switch? I mean, the only real advantage you will gain is extended oil change intervals, which is the primary reason they use it in the 97 and up models (FSS oil change schedule) You stand to gain leakage and possibly premature camshaft failure if ZDDP is in fact needed on these cars.

Given that formulations have technically gotten worse more recently (Less ZDDP) I'd stick with something close to what the engine was designed with for that era- high ZDDP.

I'm running 150K on my C36 on 15w40 Shell Rotella T year round with no usage, no leaks and no ill effects. M1 in vehicles that call for it, of course.

rjp

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03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)