Wich do you prefer and why? My section has been debateing this the last few years. My teacher, our other section leader, and I all prefer petroleum based oil, but the other 11 kids in my section are supporters of this "Fat Cat" Synthetic stuff. I like petroleum based (like pro-oil from Kanstul) because 1) it seems to last longer, and 2) it won't harm the valves. I've heard that sythetic oils will start to corrode the metal on the valves and the inside of the caseings. I don't know if this is true or not, but I figure that the rumor has to start somewhere by someone who had some bad luck with synthetic oil. I was just curious to what oils everyone else used and with one (synthetic or petroleum) you like better.