Mitsubishi Repair: Low Oil Pressure (allegedly), oil pressure gauge, oil pressure switch

Question
My girlfriend bought a 1994 Eclipse 1.8L (manual) the other day. The guy selling it to her mentioned that he thinks it has low oil pressure, due to the oil gauge constantly being low. He mentioned that he had a shop change the oil and give it a new oil filter, but it didn't fix the problem. I drove it back home, and I noticed that while driving, it went up to normal, but then after a while, went back almost all the way to the left. I would think that it might be the pump, except for the fact that the oil light doesn't come on. What do you think is the problem?

Answer
David,
You may have a bad oil pressure switch or oil gauge unit.  These attach to the oil filter housing and sit low on the car.  Over time, these can wear and cause them to stop functioning properly.  The oil light doesn't come on for low oil pressure, specifically.  It comes on when the oil level is too low; which in turn can cause low oil pressure.  You may want to have a mechanic inspect the oil pressure.  They can hook up an independent oil pressure gauge to the car and see if the pressure actually is low or if the gauges are just wrong.  If the pressure really is low, it may be a faulty pump.
Good luck!