grandpa's backhoe when it sits for awhile the oil has diesel fuel in it? any ideas on what it could be?
I think it probably is getting diesel in the oil all the time not just when it is sitting. You probably just notice it more when it is sitting since any more diesel leaking in is sitting on top of the oil or separating out. Not sure what you got there or what engine is in it. But the diesel is probably leaking out of the seals on the injection pump drive shaft. The shaft goes into the engine where it is driven by gears, etc. So if the butterfly seals leak the diesel goes into the engine and then mixes with the oil.
The engine may also have a mechanical fuel lift pump that pumps fuel to the injection pump from the tank. It is typically driven from a cam lobe arrangement. It has a diaphram in it that will eventually begin to leak, and you end up pumping fuel in two directions: injection pump and into the engine.
Just to add, I bought a backhoe once that had a failed engine because of diluted engine oil. Phil