Why would i have Oil in my air cleaner

This could be from excessive blowby.

For starters, the vehicle is 35 years old, and assuming the engine is the original engine and not ever overhauled. So we're dealing with a 35 year old turbo diesel. A vacuum leak is out of the question. Intake air enters the air filter and is sucked through the turbo, then forced into the combustion chambers. Turbos do wear out. When the turbo wears out, it can force pressurized air in both directions, towards the combustion chambers, and also towards the air intake filter. Also, a worn out turbo will leak engine oil as well in both directions. The turbo is lubricated with pressurized engine oil from the oil pump. Very common a worn turbo will saturate a air intake filter. I'd check the turbo first for wear and tear. Especially after 35 years of service.