OK then, run a search on black goo here and find out.
Once the engine is run in, it almost disappears, much depending on the oil used. Many sysnthetic racing oils wil show a complete combustion, so the oil that remains in the gas mixture never makes it down into the exhaust, no matter what the mix ratio.
My mix recommendation is 3% oil as best overall mix. When the engine is run in well, there is very little exhaust residue. I clean my plane about every 10th flight. (stab mostly)