my 1998 S10 truck with a 4 cyd engine has oil pressure of 60 psi at start up but slowly drops back to 20 psi and neqr sreo when I stop. What might be the cause?

How about a bad sensor or even a tired gauge? Put a heavier weight oil in the engine and see if that helps some. I have high mileage on my Buick and had the low oil light come on on start up. I traded the 30 weight for 40 weight and that stopped the problem. Those Chevy four banger motors were not made to last too long. They have been referred to "throw away motors" due to the soft metal used in the engine castings. That many miles shows good care. Good job, and keep on trucking. The aluminum engines will warp and change shape ever so slightly that will cause a break in some of the oil seals that maintain top oil pressure inside the engine. I had a transmission that the tail housing got out of round and the bearings would not seat so I kept losing oil from the transmission. I was told there was no fix as the rear housing could not be made round to fit the replacement bearings. "throw away transmissions". I thew away the truck.