I use Brad Penn / Penngrade 20w50 in both my old 383s. It's a blend too, I have had zero problems and nothing but good to report. This stuff's supposed to have the flat tappet additives that most modern oils don't, any more.

Only problem I've ever heard of with synthetics in old engines is sometimes the built-up sludge in a poorly maintained engine is the only thing keeping it from leaking, and the synthetic cleans that crud out and the engine leaks like a sieve. Seems to me you probably needed to replace those seals anyway?

-Kle.

P.S. - 20w50 because I donl't drive them in the Salt Season, when it's cold.