You need to run just oil or ATF. Synthetics designed for four strokes often have lubricants with teflon or other coatings that will cause clutch slippage. A synthetic oil designed for two strokes is OK, but a typical 10w40 or, I guess ATF, is cheaper and performes well. Change oil or ATF often seems to be the key, but stay away from Teflon or other synthetics that may coat the clutch plates.