I have a 99 tahoe with 70,000 and it does the same thing. Oil pressure is around 20 when idling warm. When I'm on the pedal its about 42. I remember a post while back that addressed the same issue and I think GMMerlin came on and said its normal operation. I wouldnt be too concerned about it. Now if i was traveling down the highway and my oil pressure started dropping, then I might get worried.