something weird going on with my van that i love to drive constantly

I didn't get to drive my van the week I was at drag week then for maybe 2 weeks after due to floods and whatever. then a week ago I drove over to the next town about 13 miles, drive home and notice I don't have any oil pressure.

change oil and filter. change oil pressure sender. with a 1/2 drill 0-800 rpm with a mechanical gauge, it says 50 psi at full blast but if I run the drill slow half speed its at 25, slower and its like at 0. so I'm wondering at what rpm the oil pump spins relative to engine rpm. not sure if the gauge in the dash took a crap or if there is a problem. couple days ago, the mechanical gauge was reading near 0 when it was just idling but would go up to 25 when I revved it up. totally stumped