If the needle is set properly, any glow engine will slime up the side of your plane regardless of what oil is in the fuel. Cool Power is 16-17% oil IIRC, which is less than what most model engine manufacturers suggest as a minimum for their engines (18% usually). Many folks have used it as it comes in the jug without issue, but the key is having your needle set properly.
The car guys run 8-10% oil in their engines with good longevity, but needle settings are far more critical at those low levels of oil. Higher nitro content helps as the needle needs to be richer for the engine just to run, so more oil goes through the engine at a given time with say 20% nitro than 5% nitro. At the end of the day, glow engines are all slimy. If you don't want the slime, maybe a gasoline/spark ignition engine is for you. I like the slime personally. Windex and paper towels work great for cleaning it up.