Oops, I answered your question in the other post before I saw this one.
I've gotten excellent results in my LT1 Firebird using German Castrol Syntec 0w30, along with a K&N HP3002 oil filter.
The LT1 likes an oil that is a thick 30wt to low 40wt oil, so other good choices would be Mobil 1 0w40, Amsoil 5w30 or Royal Purple Racing 41. Redline 5w30 would probably also do well. Schaeffer 15w40 has also shown excellent results in an LT1.
Mobil 1 5w30 and 10w30 don't tend to do so well in this older style Chevy small block.