Here is what I found out about the 617.95 breather system in the 81-85 300TDT:
The engine blow-by gases and cylinder crankcase vapors are routed to the air cleaner through that hose, but they end up in the air cleaner at the cyclonic oil separator which is inside the air cleaner. From the oil separator the oily gases/fumes/vapors are routed through the large intake line in front of compression and together with the intake air into the combustion chambers.
The oil separated in the cyclonic oil separator flows through a return line and check valve installed in the upper half of oil pan. The check valve prevents that the vacuum in intake system will draw oil vapors out of the oil pan.
So, you can see that the air filter is supposed to have oil in it. It should drain back down to the oil pan. You will see two tubes below the air cleaner and turbo which carry oil back to the pan. This is how you know if you have a cyclonic oil separator in the air cleaner. The other tube is the oil return line for the turbo cooling.
Both of these lines are prone to oil leaks, and I would suggest checking that check valve so you don't have oily gases being drawn into the air cleaner or turbo.