Black Soot deposit
 

I have a 1976 911 2.7 litre. Unfortunately a friend drove back from a recent trip (800klms) where I had left the car and let the oil level drop.
The oil light at no stage came on apparently, but now after a drive there is black soot on the paint around the exhaust. A fair bit of it! There doesn't appear to be a lot of visible smoke once warm. The oil pressures are good and no alarming engine noises apart from a very slight knocking. Power is still good. Do you think something has been damaged? or is this fuel related

Sounds fuel related to me. If your oil light never came on I would not worry about the oil issue too much. Generally black smoke is an indication of rich running, which might, eventually, exacerbate wear on the rings and cylinders, but mainly means you're likely to foul plugs and be done on HP. Carbs out of adjustment (needles worn, etc, or in need of a rebuild) or on injected cars, sticking or dirty injectors or a fuel system pressure regulator failure are the cause

Yup, I agree with Lee, sounds like it is running overly rich.

Thanks very much Gents
I havn't had the vehicle very long as is due for a good tune, so lets hpoe that does the trick.