There are a few options. Personaly, I like mobil 1. Synthetic is the way to go IMHO.
Problem is, you can't just go dumping synthetic in. The synthetic has so much detergent that it will loosen all the gunk deposits in your engine possibly causing the oil passages to clog.
I ween my engines onto the full synthetic by using regular oil and substituting one qt. of mobil 1 and change the oil again in only 3000 miles. Next I refill with 2 qt of mobil 1. After about the 3rd time I go all in.
Besure to use a good oil filter such as mobil 1, K&N, AC Delco gold (or something like that).
One other thing, synthetic has a way of finding oil leaks you never knew you had. The gunk may have been cloging a leak and the synthetic will clean the gunk out exposing said leak.
If the engine has had frequent oil changes from day one and does not have any sludge don't worry. Use any oil you want.
Only heavy sludged engines that were "pushed" on oil changes may exhibit some issues using a thinner synthetic oil, Oil leaks, ETC... Fix the leaks and you will be fine. What Small block does not leak oil anyway?
Any on sale name brand oil was used on some of the cars I drive for over 10 years. Now I use synthetics. The oil was always changed frequent so the engine looks new inside. Oil leaks? Sure.. I fixed most of them. The engine did not leak any worse than it did before I did the switch.
Ive used the standard grade of M1 and castrol syntec (5w-30) also Diesel oils in 5w-40 weight. (Mobil 1 and Shell Rotella) When German Castrol goes on sale, 0w-30 I might try some of that.
On that note...I've bought many "high mileage" cars with questionable oil change history (police cars, old peoples cars like my Camaro once was)...and dumped straight up full synthetic right in for the first change (my synthetic of choice is German Castrol).
I've NEVER had an oil system issue on any of my cars, and any engine I've lost, has not been a fault of the oil. My only reason for running GC, is that I can stretch it until the change oil light comes on on the dash (which is about every 12K miles in the Camaro) and I still don't wind up with any sludge or buildup in there...if you're in the "OMGzzz I have to change my oilz every 3000 miles or 3 months religiously" camp...then you can run just about anything you can buy off the shelf in a parts store in a typical gasoline engine like a stock LT1. If there are no oil pressure issues already, I'd continue to always run 5W-30 in it...thats all I'll ever run in New England...where it gets up to 100 in the summer, and down to -10 (Fahrenheit) in the winter.
I run the semi-syn stuff, always been a little leary of full-syn, but that is just my old school thinking ......change it every 5-6k....looks and smells about the same as when in pored it in...I could probably go 7-8k.