If you think this is happening, you should go to your dealer and ask them to print out the diagrams of your tractor engine and pump. Mine does this for me all the time. They never charge me anything to do this. I think they would rather have me do the repairs, so they do everything they can to help me out.
While you are there, you should talk to the mechanics and ask them if this is possible. I've been trying to figure out how it could happen. I know that an engine can leak oil out out of the ream main seal, around the crankshaft. This would be the only way anything could get into the engine if it was coming from the hydraulic pump. I just haven't figured out how the hydraulic fluid could work its way into the engine.