Level indicator

[Don42]

Or a magnet on a doughnut shaped float around a tube with magnetic reed switches at different depths that would light an LED for you.

This is an excellent idea, better than the ultrasonic approach that I suggested. Simple and cheap. He'd only need one reed switch if he only cares about a "go-nogo" indication. Reed switches are hermetically sealed and made of glass so they can tolerate immersion in oil with no problem. Sliding sensors like this can be problematic in water because of mineral buildup, but should work very nicely in oil. Things in a oil tank can get cruddy too, but not like in water.

Well done, Sleykin!

nhengineer and Sleykin: I'd be glad to correspond. I have both reed switches and magnets in my goody box and I'm willing to share. I'm a widower since 2011 so I keep very busy, but I check email often even when travelling. I'm outta this MN icebox for the FL keys next Monday for some bluewater fishin', but I'll have my laptop and I do keep up with email. If y'all care to correspond, pls use this addy:

dforeman2x3@goldengate.net

Delete the 2x3; that's there only to deflect spam bots that don't actually read the material they scan for email addys.