Been using Delo 400 HDEO in my brother's modified 283 (302 sized) '67 SBC, running mechanical lifters, long slot rockers, Edelbrock Performer 600 CFM carb, not an extremely wild lift cam (double valve springs, though), combined with Comp Cams zinc aditive. Recently adjusted valves after break-in, they weren't very far off, only a little tightening needed. He had a 383 stroker motor in the CJ-7 originally, but the PO ran it too lean & actually damaged the block & one head pretty badly, block beyond repair. Your builder can tell you what bearing tolerances you have, that will determine what you run-but thicker seems to be generally better, to a point, when you race and/or run carbureted (which tends to get fuel in oil).