Two years ago, my truck was running at a fast idle and I could not
find out what was wrong. I took it to a mechanic and he corrected the
problem by replacing the oil sending unit. He said something about
oil leaking out of the sending unit and causing a short in some wire
that apparently had something to do with the idle. I have no way of
contacting him so I wanted to see if anyone can explain what was going
on that was corrected by replacing the oil sending unit. The truck is
back to idling too fast. Any insight will be appreciated

When motor is warm, remove cover to the air cleaner and check to see if the choke is vertical. If not, the choke linkage, choke heater, or voltage from the alternator is at fault. Voltage from the Alternator is wired directly to the choke. The connectors can get corroded at the alternator. Some of the dash lights may be dim or out completely because of the corroded connectors.

There is also a pressure detent that holds the throttle high for a short time to clean the exaust while retarding timing. If this is to stiff or the linkage is binding it will hold the throttle speed high.