Try replacing the seals for the turbo lines. I'm no Benz repair guru, but of all the leaky w123 turbo's that I've seen, the turbo lines are the culprit. Plus, at 250k miles, you should replace them anyway. Also, make sure that the line that connects to the bottom of the air filter housing is actually connected to the housing. This line is fed oil vapors via the breather hoses from the valve cover. Also check the waste gate hose and the upper and lower turbo seal rings. When I bought my car, the waste gate hose and lower seal ring was hard and cracked and the upper seal ring was not installed properly. Once I replaced these items, the car performed better and the small oil leaks around the turbo stopped. The previous owner had replaced the seals in the turbo lines already.
'82 300D Turbo 215k miles