Well after installing a new sending unit on my 96 12v my oil pressure gauge stopped bouncing around randomly. Now it just shows 36-37 pounds at idle and about 40lbs cruising. Even the faulty sending unit showed higher than that. (about 42 at cruising) Soooo..... I installed a mechanical gauge off the oil filter mount housing and the mechanical gauge showed 74lbs @ 750rpm and pegged the gauge at 100lbs when I blurped the throttle. Is the factory dash gauge really just there for comfort? I mean that was considerably off.

 

My opinion of the factory gauges is that they are one (small, very small) step above idiot lights. They are only there for comfort. Even on yours the computer is what controls the gauge. It is programed to put the gauge in the middle no matter what providing you have any oil pressure.
Tom

 

This is why I have been throwing around the idea of adding a Mechanical water temp. gauge measuring off the #1 tap, and a oil pressure gauge off the housing like you did. I just dont trust the dash board.

 

The oil pressure at the filter housing has always been 20-40 psi higher than at the stock location with a mechanical gauge on every 12 valve I've measured. I think the block sender location being closer to where the oil starts dumping into the pan causes this. If I were to hook up a mechanical OP gauge I'd put the sender where the factory one is rather than on the oil filter.

 

I have to agree with infidel were to hook up the mechanical oil pressure guage. I would install a tee so you still have the stock guage and the mechanical guage also.

 

I teed a mechanical guage into where the stock sending unit goes. The factory guage is a joke, it reads different each time the truck starts. My mechanical guage reads about 28 at idle and a constant 55 psi running down the road. The factory guage is for show, DO NOT trust it, and if I remember right, the factory sending unit reads pressure after its gone thru the motor.