the erratic gauge almost always means bad sending unit (on any car). I would replace that first. If it does not help, test the gauge next, then go to a mechanic and for $25 have them test the actual oil pressure. Sure beats killing yourself guessing.
1985 190D 2.2l Sold-to Brother-in-law
1996 Mustang 3.8l -"thinks it's a sports car"
1988 Grand Wagoneer - Sold (good home)
1995 Grand Cherokee Ltd -"What was I thinking??!!"