I've used STP for years putting engines together and not had a problem. The 261 in my Chevy was put together 25 years ago with STP. Maybe just dumb luck on my part. Otherwise,I don't use oil additives. I just keep my oil changed at the recommended intervals.
I do like the Lucas fuel additive. Seems to help keep the electric fuel pump in the tanks lubricated, but on my carbed engines, no fuel additives either.
