Hey everyone, this is my first post here, just picked up a SS427 custom coupe , 1968, and I am wondering if synthetic is alright for an older engine, let me rephrase that . older style engine but fresh.
I really can not offer any advise as I have never felt compelled to pay more than necessary for my motor oil. I have been using petroleum based (refined crude) since 1963 without an issue. I have read that in an older style motor (two piece rear main seal with perimeter valve covers you are more prone to leaks with synthetic, and I have not read anything other than sales hype that would convince me to try it.
Of course I don't own a smart phone nor will you find me on Twitter or Face Book, and I haven't attempted to build a hot rod with EFI as of yet. So you can take my rousing lack of endorsement for what it is worth; free advise.
It should have 70W90 gear lube inside of the Muncie not motor oil. If you can find it 85W140 would be even beter if available. It should list the API rating of GL-4 on the bottle (GL-5 is now out), but as I mentioned for the past fifty years I have been using Valvoline gear lube and I have built (or destroyed depending upon your point of view) a lot of Muncies four speeds. I had to; as I made most of them into crash boxes and a lot of my customers couldn't figure out how to properly shift a crash box, or else they tried to run one on the street.