My oil pressure goes down as the hummer gets warm?

Used 2005 hummer H2 SUT with 57K miles. Recent oil pump and water pump replaced due to sludge in the pan and lines. Oil pressure was below 20 and running hot when the replacement occurred. Now on start up it is 40 pounds and after 15 minutes it starts fluctuating between 25-40. What gives?|||when car gets warmer, oil gets thinner.|||I would install an engine oil pressure gauge on the engine to check your dash gauge which could be reading erratically due to a gauge or wiring problem. You can tape the shop type oil pressure gauge to your outside windshield and drive the vehicle to see what the actual engine oil pressure is really measuring.|||It's normal for oil pressure to drop a little as the oil heats up and thins out, it's also normal for it to fluctuate with engine speed. If the fluctuation is all at idle you may have a bad sending unit, or there may still be some sludge left in it. If it just falls when it gets warm and stays low you may have some main bearing damage.|||Hmm Possible rod and main damage. It should drop some. But this sounds more serious than that. Possible a cracked block here. Why did it have sludge in the pan? This should never happen.