I use Joe Gibbs conventional, I think 10w-30. The Joe Gibbs thing is for the appropriate level of zinc/ZDDP for a flat tappet cam, the conventional is because I don't think synthetic is needed or even a good idea for our motors (they weren't designed for it and synthetic
may leak past the looser seal tolerances). It's expensive, but I only change oil in my vette once a year.
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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.