Because the oil pump is engine driven, the oil pressure rises and falls in direct step with engine rpms. 20 lbs. of oil pressure at idle is very good, and it rising higher at higher engine RPMs is normal. A good rule of thumb for oil pressure is acceptable 10 psi per 1000 RPMs.
On newer TJs, that is not the case since newer oil pressure gauges no longer indicate the true oil pressure. They tend to move to mid-scale and stay there so as to not unnecessarily alarm those who don't know how oil pressure is supposed to work.