The oil pressure sending unit is at the front of the engine near the oil filter. The fitting is most likely 1/4 NPT.

 

You can check compression if you wish...not a bad idea...warm up the engine pull the plugs out and hold the throttle open and bump it over the equal amount for each cylinder...do a dry and wet test. Wet test is putting oil in the cylinder to see if the #'s increase. If they do it's the rings.


Low oil pressure could be caused by a failing pump or clogged screen but when an engine is old it does things to the oil pressure that form a pattern.

When the engine gets old and the bearings wear...the gaps and spaces drop the oil pressure. When an engine starts getting like 10 or less psi at hot idle...it's getting time for a rebuild in the future. When it's in the low single digits it's about time I'd say. The goal is to get it before it eats itself so that you don't just ruin more parts in the engine. It may run like that for awhile or it may not...who knows...but I personally make the cutoff around that point.

If the pump was bad you'd have bad oil pressure above idle...same with a clogged screen. When the motor is old and you've got low idle pressure...it'll still be alright at higher RPMs. Two different senarios.

 

i see motor is probably just getting tired. Hopefully can sell it or if worse comes to worse pull the 351 out of my 1990 and drop it in there and then sell it. Then i'd turn my 90 into a trailer. the rest of the 88 is in decent shape some rust not to bad yet be better if the paint hadn't fallen off. Tranny,t-case and front end is still good shape.