Jeep Repair: oil pressure, oil pressure gauge, jeep grand cherokee

Question
We own a 1994 Jeep Grand Cherokee with 216000 miles on it. It had about 40000 miles on it when we bought it in 96. Lately the oil pressure gauge has been going haywire. When you get at or around 60mph the gauge pegs at 80psi but as soon as you slow down it drops down to about 40psi. When driving in town the gauge jumps around anywhere from 10-60 psi. It just started doing this and since I've noticed that the fuel mileage is way done. Can anyone hlep? Thanks for any info you can provide. Jeff.

Answer
I would replace the oil sending unit, It may be causing the reading to be off.  It is easy to replace, it is just above the oil filter on the passengers side.  If your reading are still different I would suspect an oil pump.  I don't know why you gas mileage would be affected.  Hope I could help you!