Assuming this vehicle is fuel injected, you can do a leak down test.
Someone is putting it there. There is no way for oil to get from the engine to your fuel tank by natural means. Literally, if you have oil in your fuel tank, someone is sabotaging your vehicle by putting the oil there.
The best solution to your problem is an in-frame overhaul. If fuel is getting into your oil, or oil into your fuel, you've got worn piston rings.
Getting rid of an old oil tank is a difficult task. The oil will have to removed first. You may be able to donate the oil. The tank must them be cut in half in order to get it out of the house. Call your local trash collection company to find out where you can take the tank for disposal.
Leaky fuel injector(s).
At the very least, to fuel your Hummer.
Coolant getting into the fuel or oil.
Oil. It is becoming more difficult to find and produce oil. The oil boom is over.
If air or fluid was getting into the oil passages. Is the oil looking "bubbly" or "milkshake" like? Then coolant is getting into the oil, either by head gasket or somewhere else. If it smells gassy then fuel is getting into the oil.
if you have a mechanical fuel pump on the side of your block, the diaphragm has a hole in it which will dump fuel straight into your oil. a low cost fix.
needs a new fuel pump
Dry cylinder has 2 definition 1 no fuel is getting to the cylinder 2 no oil getting to it
The fuel-oil mixture enters the fuel-oil separator. Since the oil is denser than the fuel, by centrifugal separation, it will separate the oil and fuel. the fuel will be on top and the oil will be at the bottom.