No oil presure, what do I look for

Sounds like the damage has already been done and the new, thicker oil hides the excessive bearing clearance until the additives start to wear out and the oil becomes thinner. Also check the oil level. If it goes up over time, you could have a leaking injector allowing raw fuel to run past the piston rings into the oil where it dilutes it and reduces its lubricating properties. GM has had some trouble with leaking fuel pressure regulators too although you should have noticed severe engine performance problems.

I would start by removing the oil pan and a few bearing caps to see what the bearings look like. The oil pump could be weak too but it's more likely to find worn bearings or a broken pressure relief valve spring in the pump. Worn pumps seem to be less of a problem than other things.