could be the sender or gauge. Why not try a thicker oil ? 20w-50, maybe 3 quarts and 1 1/2-2 quarts of 10w-30 cause winter is coming. All oil with the W can be used in winter--least how it used to be.
if its got 5w-30 right now, I would drain it--leave filter on and pour in 4 quarts of 20w-50 and see what happens. might keep it going for a while..
But first if not too expensive, change the sender
My 92 olds reads too high, sure the spring in the sender is bad--needle is peg till engine heats up..been that way for 5 years now, too lazy and cheap to change it.
Motor Honey thickens oil also