If the engine is new... The first few tankfuls will leave some residue in the form of small black spots...I believe this is due to the wear and seating of the internal parts.. As the engine breaks in, they will gradually disappear... I run 40:1 synthetic oil, and there is no cleaning after flying......In a short time, you will be wondering why you waited so long...cheaper fuel, super reliable idle, no worry about tank placement, etc......