New one to me.
Is there by chance a fuel pre-heater that uses the warm engine oil?
Look at where and how the fuel filter base mounts to the engine. Any oil passages in the block close to this area?
Have you called your CAT dealer? Try to figure out a way to have a conversation with an actual mechanic. Be creative.
I don't know about your particular engine,but it happens a lot with the Mitsubishi 40M0 engine.It's caused by the input shaft seal leaking - the lift pump is at the front of the rotary injector pump,and instead of diesel leaking into the sump,it pulls engine oil into the pump....then back to the tank by the return hose.
I'm with Lada on this, but if you have enough oil in the fuel tank to be able to detect it and it is pulled in through the pump, shouldn't you have been having an oil comsumption problem as well?