Normally all synthetic oil doesn't reduce the oily residue left over. The helicopter guys tend to like it as with their engines they have less residue left over to clean off of their heli's. But the reason there is less residue is due to the fact that the synthetic oil does burn and is consumed when the engine is running. Thus little to no residue. But the scary thing is if the oil burns, then what is lubricating the engine? Now they can get away with it with the helicopters due to how they adjust and run the engines. The heli guys run their engines fairly rich. You can see the cloud of smoke coming out of them too. Except the oil in their case is burning and thus the smoke. But for model airplanes, when you have the engine consuming the oil then it is too lean and it burns up quickly. So you need to be more careful to ensure that the engine doesn't go lean on you.
Now then I did not see any difference with leftover oily residue on my planes using all synthetic versus castor oil or a blend for that matter.