"Pontiac recommended that the HO engines use straight 30W in the summer." MEANING WHAT... 0W-30?
sorry about the caps...
Not sure I understand your comment but this is what I've used for decades in my 400HO, straight 30W as noted in the Owner's Manual, not a multiviscosity oil: (Plus, now I add 4 oz of ZDDP to replace the zinc that has been removed from the oil of the 60's-80's)