The oil pressure doesn't change, but the reading on the gauge does. The oil pump is still pushing the same amount of oil thru the bearings.
All due respect but I don't think that's true. If there is less force applied to the oil in the pan pressing it into the pump, the pump wont draw as much oil as the pump creates a negative pressure to operate. The question is how much does it effect it ?
For info 14 inHg = - 6.88 psi, negative OR 7.82 psi of pressure in the crankcase / pan instead of 14.7psi (at sea level).
