oil leak or blowby?
 

My 83 turbo diesel has oil on the valve cover after a drive. It comes from the filler cap and the hose from the valve cover to the air cleaner. A blowby check with the filler cap gives a typical "tea kettle" movement which I understand is fairly normal. The mileage on the motor is unknown. Do I have bad seals on the cap and hose or is it a blowby problem?

yes and yes
 

probably some blow by and needing new seal for cap and clamp for hose.

good luck

tom w

Your car may still have the old style breather hose, mine did when I bought it a couple of years ago. The updated hose is a one piece rubber unit, versus the plastic and rubber two piece original. I would replace the oil cap seal every other oil change also. It helps keep the valve cover clean. One shop in my area changes the oil cap seal at every oil change. Good luck.