It is very common for the oil return line from the turbo to leak on the 617.9XX engines. The seals should be avilable at any good parts outlet that handles MBZ parts. I am sure you can order them right here at Mercedesshop.com. I would repair the oil return line first. If it doesn't "fix" the oil leaks then I would search for other oil leaks. Don't forget about the valve cover gasket. I had a shop adjust my valves and they kinked the valve cover gasket. It leaked oil on the left side of the engine under the air cleaner.
The procedure for changing the oil return seals is availabe on the internet. I would search the archive here first and then search the general internet.
79 300 SD