As Marv said, about any float that isn't eaten by oil will work fine.
I think Delrin will float on oil. I have lots of that. I'll check out the density and compatibility with fuel oil.
I have lots of different magnets that would work fine.
This magnet to be attached to the float, right?
You would want something on the bottom of the tube to keep the float from falling off the end if the level got too low and also so the float won't drop below the last reed switch.
The reed switches would be normal open switches. One switch at 1/2 full and another at 1/4 full. The one at 1/4 full would be the bottom of the plastic or copper tube and would stay lit until the tank is filled.
Lit? Wouldn't the bottom reed switch be NO until the floating magnet was near it?
The circuit would be a simple 3 wire 2 AA batteries would operate it just fine.
So, all you would need are 2 reed switches, 2 LEDs, the float and magnets and a tube and some wire.
Why 2 LED's. Would I only need one (red) to tell me when I'm at or below ¼ tank or would the other one (green) be to tell me that the device was operating properly?
I was thinking 3/8" tube would be ideal and the float would not have to be a large diameter. Do you have a lathe or drill press?
You could easily make a float out of anything floaty.