If you have a coated piston it is usually to aid in break-in and in my experience has nothing to do with the oil type . There are quite a few "coatings" used today. Synthetic or petroleum should make no difference? Unless the OEM supplies you with oil to use, they can not tell you that they will void your warranty because you used the wrong oil. If they give you oil then tell you if its not used it will void the warranty they have ground to stand on. Now....... if you decide to use brake fluid when they recommend a "good quality" 2 stroke oil in their manual you may have a problem. I will also say that if an OEM asks for a failed part for testing and you used something that you shouldnt have used ( like brake fluid ) they will find it, we always did. Just be honest and follow the guidelines and, Oh yea, deal with a reputable manufucturer. In the end they want you to be happy so they will make most problems right. If they dont then you shouldnt do business with them.

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