RLFGary,
The subject has been very heavily debated! Look out with a question like the one you posted!! My experience is that every engine is different how they spit the oil out. The only way to find out is to experiment a little. I have an O.S. 1.20 AX in a pattern plane that has used both fully synthetic Cool power 18MV, Omega, and Byron 18% synthetic castor mix. Guess what, they both make an absolute mess on my plane. Could it be because of tuning? You bet!! And whole bunch of other factors. My experience has been that a castor blend seems to leave a more noticeable slime. But, what really counts is how the engine looks inside and how it runs. If your not burning plugs or running to lean just go fly and have fun. Nitros spit all over the place and there is nothing you can do about it. Try even angling the muffler a different direction to help you out maybe...that helped me.