1989 Chevy Suburban oil pressure gauge

I have a 89 chevy suburban that after about 10 miles of driving or so the oil gauge drops to 0 at idle in drive only if you put it in park it stays at 15 and when your on the gas in drive it stays at about 18 when you start it and for the first 10 miles the pressure it right at 30. it runs pretty good but after the pressure drops if you try to start it, well it takes a while whats my list of things to check and replace first.
thanks
brian