I don't know what synthetic is, But I have used almost every type and brand in the store. I own a 96 GMC Burban and its got 220,000 miles on it and other than the first 20,000 miles it has had Mobile 1 10-30. Still on the original motor and it doesn't smoke one bit. I"m not sure but others were saying its an old motor and it was built for 30w, but if your rebuilding it its probably gonna be alot better than stock so I'd use a type of synthetic. You just can't use synthetic during break-in...it works so good the parts won't mate up. I also have a 65 cutlass with the orignal 330 with well over 200,000 and I've been running Mobile 1 with out any problems.
