temp & oil gauge not working

I've seen and serviced a few hundred of these thermal instruments.
95% of fuel gauges toasted ( due to their function and duty cycles over many years ), 60% of oil gauges toasted, 20% of temp' gauges toasted.
Here's a somewhat common scenario that sheds some light on that oil gauge percentage.
The original terminal for the oil sender looks very much like a female spade terminal. It slips over a button like contact atop that sender. If the female terminal had a plastic isolator on it, it didn't last very long ( it's a rare sight today for sure ). This bare terminal would often get bumped off the button like contact and land against most anything close creating a short to ground.
A 0 ohm resistance path isn't good for a 80-10 ohm instrument. sigh
Todays solid state regulators produce more stable output voltage at much lesser amperage so a short circuit at sender side doesn't fry an instrument nearly as quick as the OEM mechanical limiter could/would.
Even though the output voltage is routed to 3 instruments, current always takes the path of least resistance. We can't begin to guess what resistance the fuel sender and temp sender offered while the oil sender wire offered 0 ohms.
In the end I've typed a few facts that may or may not explain what your diagnosis reveals. Good luck