I own a 328 and have been working on the professionally since they were new cars.
I use Redline 5-40. Any good quality synthetic 5-40 will do.
I am a believer in usng premium fuel in any motor that will be run hard. It just give a better margin of detonation protection if someting isn't perfect. Also no gas in the US, from coast to coast is as good as it was in the 80's.