I have always used mobil 1 5w30. I was told you could even drop to 0w if you wanted to. Also, instead of adding that 5th qt of oil, add lucas oil treatment. **** works, believe me.
I may try Lucas on my next car. It will have my stage 2 block in it. I've just been using what the GM says to use on my current car. I was just wondering if there was something better then GM specs. Thanks Tim