Why is the oil pressure lower on 1999 Chevy Tahoe after engine heats up its at 40psi when you start the truck after heating up drops to 20psi?

Answer 1

Because of the way the oil reacts with the heat and the oil viscosity changes. it is a normal occurance and the specs you state sound perfectly normal for an idling engine. the oil pump and oil regulator valve are controling the pressure and as the oil heats up it gets thinner and hence the lower oil pressure. pressure goes from high at start(cold) to lower at operating temp. (ever notice that cold oil pours slower and appears thicker than hot engine oil) you can not just add thicker oil because during start up it must flow to lubricate everything, as a lot of engine wear occurs at start up.