I'm just about to re-assemble my '69 396 with Muncie 4 speed. In my 4th gens, I have been using synthetic fluid in everything and I would like to do the same in the '69. I'm speaking of Mobil 1 75-90 for the Muncie and the 12 bolt posi, synthetic for the power steering and brake fluids, and of course, Mobil 1 for the engine and Dexcol antifreeze. Anyone have experience or know of reasons why the "old stuff" won't perform well using synthetics. Thanks alot.