Turbo Pressurizing Engine?

[Crowe]

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?