Oil pressure electric sending units usually electrically 'ground'
through the sending unit. The trandsducer varies how much these units
ground by variable resistance. If the gage is showing zero = open
short; if pegged at high pressure = dead short .... Corrosion of the
'connector' on the sender will alter the resistance. Such senders
are quite cheap; do a resistance test on the wiring, then ultimately
replace the sender and see what happens.