The causes of oil pressure fluctuating in a vehicle can be caused by a few different things such as faulty oil pump or faulty by-pass valve.
Other causes of oil pressure fluctuating are clogged or partially clogged oil filter or too much oil in the engine.
It's normal sometimes for oil pressure to go up and down on the vehicle especially when the engine RPM goes up as you're traveling down the highway.
My vehicles oil pressure ranges from as low as 45 to 50 PSI and then sometimes when I'm traveling at 65 to 70 MPH the oil pressure goes up to 65 and sometimes 70 PSI which is okay as well.
If your oil pressure goes up to 80 PSI or above and doesn't go back down then it could be a bad pressure relief valve for the oil pump or could also be the clogged oil filter.
If you haven't changed your oil filter or engine oil in 5,000 miles or even 3,000 miles then it's time to change the oil which could be causing the oil pressure to fluctuate.
You should change the oil filter as well every time you change the oil because the oil filter needs to be clean and not clogged so that the oil can flow through the oil filter and engine properly.