On my A2 truck, this was happening due to rusted air piston on the transfer case...IIRC, the air piston at top rear of transfer case was rusted to the point of preventing it from engaging, once rebuilt with rust-free components, it worked & shifted fine and I haven’t had any more oil in my air filter housing.
If you check on the back side of your intake stack, closest to frame rail, you’ll feel 3 or 4 vent lines going into the intake stack. Disconnect them one at a time and try to find the one that has oil in it, then trace it back to its origin to start troubleshooting.