I've got an original engine and only run chevron 94 octane (Highest available) with no ethanol. and it runs great. I have not heard of the oil additive though. I'll have to see if I can find it. Will stores know it by ZDDP?
I've been using this one for a few years now:
http://zddplus.com/The zinc and phosphorus that was in oil has been phased down to low levels because the EPA thinks it harms catalytic converters. Our flat tappet engines need it to keep down the wear from the lifters and cam lobes.
Have not seen it in retail stores. Amazon, EBay, and some of the Pontiac vendors have carried it.
