with that kind of number on your odometer i'm not suprised that you get low oil pressure readings at low revs. what type oil are you using? if you have 5w-30 or even 10w-30 then switch to something heavier like 15w-50. that'll get your oil pressure up some. in a car with this many miles, it's not going to be any single thing. oil pumps, main bearings etc. all form a delicate balance and unless something is really broken you shouldn't upset the equilibrium. i would even try that oil additive called "restore" which addresses many of the issues and will keep your car going for several thousand more miles. of course, the best way to know is by having the actual oil pressure read off a gauge to determine the real situation and maybe it really is just the sending unit...