have a 87 300SDL and there is a very small cut / opening in the exhaust housing next to the turbo. Is this something that can be fixed, also oil residue is on the turbo.

 

It is kinda hard to know exactly where this cut, or whatever it is you see, without posting pictures!

If the cast iron manifold is cracked, you may be better off replacing it. Turbo's will weep a tiny bit of oil in these 603 engines and that is not a bad thing, or unusual. If it is constantly dripping oil and you have to add oil frequently as a result, then there is an issue with seals, or even a loose oil supply tube.