one more thing
at NITRO engine when you have black oil residue it means that the engine is about to end he's life - or one of the main parts such as sleev/piston/ring etc'.
somtimes it is the the crank shaft rear part scratching the back plat.

in GAS engine this can be the issue as well, assuming that the engine is new and have no factory/manufacturing screwups i would suggest to try and replace the oil type.

you got a 400-800$ ARF with another 400$ new engine and another 300-400$ RX+servos, why not go and buy the best oil money can get??? for 1L it will cost no more than 20-30$ TOPS.
both oils the petrol; and sync' ones - go and get the best there is.
i belive that this will reduce the residues that you see.


good luck

shalom from sunny Israel and Happy Passover .