Gas trouble 199 grand cherokee

My 1999 Grand cherokee laredo started to use up dramatically more gas last 6 months, down to 8 miles per gallon or less, and its oil pressure gauge moves up and down with RPMs. Couple mechanics gave me much different solutions. What should I do?

Thanks in advance for your input

First, make sure that the excess gasoline is not getting into the oil. A broken fuel pressure regulator valve can drain fuel into the intake manifold and from there to the crankcase which may explain the oil pressure fluctuations with RPM. If the oil level is high and smells strongly like gasoline have an oil change done immediately.