i have had a lot of experience with ethenol in my boat motor(all bad). this stuff is junk!! go to trailer boats magizine and look for an article titled (maritime moonshine). you will get educated on this junk! 87-89-91 oct, all contane ethenol, it has nothing to do with octane rating. ask around your area for non-ethenol gas, it is still available. if you find it i defenitly recamend you use it! cost a little more butt worth it. some stores sell it as recreational fuel. regular sta-bil does nothing for ethenol, make sure to get sta-bil marine formula-ethenol treatment. remember ethenol is an alcohol product. it is a solvent,eats up rubber fuel lines, seals etc: please read the article! it tells the truth! briggs n stratton makes an ethenol tester for under 10$. get one, then you know if the gas station telling the truth about the e-rating. as for which oct to use, use 87oct, then add lucas octane booster(good stuff). save money & get the power too! i hope my misfortunes will be your gain. READ THE ARTICLE! good luck.