That would depend on what kind of fuel system the engine has. Engines with a carburetor/s have a float bowl that contains fuel and floats that operate a valve that the fuel passes through into the bowl. If the floats stick open, raw fuel can seep into the crank case past the rings. Engines with fuel injection can have leaky or worn out injectors causing the same problem.
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.
Assuming this vehicle is fuel injected, you can do a leak down test.
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.
Leaky fuel injector(s).
At the very least, to fuel your Hummer.
Coolant getting into the fuel or oil.
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.
Oil is a fossil fuel.