I had the same problem with my evolution PS 46. I was using fuel with 16.4% oil content and 4% nitro, when I noticed a black residue with the oil exhaust. I try to increase for 18% of oil content but the oil exhaust continued black.

Second Dave Gierke, the black residue is from piston wear due the little oil content in the fuel, that is not giving the proper lubrication to engine. This engine should have a minimum of 20% oil for lubrication. Also, for a maximum performance with this engine; use at least 10% nitro.

Them I increase for 20.5% of oil content... the black residue stopped and the oil exhaust is clean.