seems like a common problem it happened to my 89 4.0. I droped the pan installed a High volume pump, problem solved for now. 20 psi at idle in gear 45 running at highway speed. I know it is obvious that it is due to engine weare cause of high mileage but a tare down and rebuild is not in the budget. spending 20 bucks more on a high volume pump over a stock pump is well worth it. i tell people all the time not to cheese out and get a stock pump. some people choose not to listen and after putting in a stock pump the have either very little improvement or none at all.

 

Well im just going to say what i got to say IMO the sending units are crap plus the gauges just give approximate pressure by way of electronic pulses your gauge may say zero and the sending unit may be good but you could actually have 10 PSI just like as was said before you could try a new sending unit but I changed mine and a new gauge and new oil pump and when i got a good autometer setup the difference was amazing like by 20 PSI but that's just my 2 cents and you know what people say about opinions

 

mine does the same thing at idle sometimes drops to zero and at driving atmay have 20 and the whole top end taps its been like that since i bought it and since then i have put 7000 on the motor and wheel it every day just drive it till it blows thats what i plan on doing the motor also has 210000 on it