I'm currently using Miles Syn-Blend 5w-30 in our '89 Camaro RS 305 TBI. It's API SN/GF-5 certified. I bought 12 quarts of it for $14.08 on Amazon a couple weeks back.
We recently took the Camaro on an ~300 mile round-trip from central IL (where we live) to Le Claire, Iowa, to go see the American Pickers store, and other tourist shops in the town (it was a really fun trip and I would definitely recommend anyone willing to drive there to do so). The Camaro ran great (except that the brakes grabbed and liked to pull left).