Turbo Pressurizing Engine?
 

Is it possible for the turbo to leak pressure inside the engine? I am experiencing an oil leak on my ’80 300SD after the car has been run at interstate highway speed. The leak appears in the rear main seal area of the engine. If you drive around town or on short trips the oil leak does not appear. However, the leak is significant if the car is driven at highway speed for an hour or more. When this happens I also experience a noisy and rapidly vibrating oil pressure needle at idle. Could the turbo be causing pressure inside the engine? If so must the turbo be replaced or is there another fix? No other symptoms. Any thoughts?

On the highway oil tends to get hotter then around town. Hotter oil gets thinner. As oil thins it can leak from places it couldn't leak from when cooler.
my .02

Oh yea, and higher rpm on the tach also means higher oil pressure, and that will pressurise your leaks.

When the turbo's blowing at full tilt an increased amount will blow through the rings (blow-by) and increase crank case pressure. A crankcase vent system in working order will keep things in check. If your rings are worn, the crankcase vent system can get overwhelmed. I don't know if this causes oil to blow past the rear main seal, but you'll see oil seeping past a less than perfect oil filler cap seal.

Sixto
95 S420
87 300SDL

Here is a couple of things I ran into:

When I bought mine, the air cleaner mount was broken. Consequently, the o-rings on the bottom of the oil separator were dried out and cracked. When I replaced those, I seemed to be having a lot of seepage.

I found that the drain from the oil separator to the oil pan was plugged! The mount must have been broken for some time, years? I ran a length of wire down through the line and drained the oil. Then I poured engine flush (about the only thing it is good for) through the drain tube only.

Since then, I have had very little seepage and the air cleaner does not have the major build up of oil anymore.:)