I am not sure about the factory gauge dropping that much, but since you just bought the car. I'd check the oil first and look at it condition( color, viscosity, gassy smel, etc..) then probably change it and do a complete tune up
Oil pressure in the low 20s(when warm) is usually from loose clearences on the crank, alot of fuel in the oil will also cause low oil pressure as well as some oils. I find synthetic oils to hold the pressure higher over a longer period of time.
[B]95 GT Now 331 TFS TW FoxLake stage 1 heads, TFS-2 cam, Track Heat Intake, 75mm TB, ProM80 MAF, 1-3/4" mac Long tubes full 3" mac exhaust rwhp 351.65 rwtq 370.71
95gt On stock engine email@example.com 1.765 60ft 8.567 1/8th mile w/slicks
firstname.lastname@example.org 1.81 60ft w/slicks
Also had 87Gt carb'd 306 best time email@example.com N.A.
88 Ttop GT(rare)w/ lots of mods