The oil that is in the fuel for lub and cooling due to gravity tends to start to collect in cyclinder at lower rpms. At full throttle there is enough exhaust to blow it out the muffler which is why CL engines whcih run at full throttle for the full flight don't seem to have a problem. When the oil collects at lower rpms, it will start to douse out the glow plug and you will lose power and eventually the engine will quit unless you throttle up again. The 4 strokes operate differently and once started are pretty trouble free even when inverted.