I have heard from more than one person that switching between conventional and synthetic oil can do serious harm to or drastically shorten the lifespan of an engine. My father also told me that when mixing oil, if the engine manufacturers instructions differ from that on the bottle, follow the engine manufacturers instructions. I have had four engines. One of them snapped a mounting bolt after a couple months and I never used it again. One was a centrifugal clutch and the one way bearing failed. One lasted a year before the rings went. My current engine is almost a year old and runs like a champ. I have always followed the engine manual's instructions for mixing, regardless of what the bottle said. I have never had any problems, except for my first engine, with the rings going because I was using marine oil. I now use air cooled oil, and any air-cooled synthetic oil will do the job. I currently use universal 2 cycle synthetic oil which I purchase at walmart. For how cheap these engines are to replace, I say do what you can to get it running the way you want, run it till it dies, then get a new engine. Thats my two cents.