Gas
I've got a 1999 Thunderbird 900... I run 87 octane in it. I've also got a 2002 Bonne America... went on a camping ride with my wife last summer in a good rain. She was on the Bonne and it started running really rough, especially at startup and in low gears, after we filled up on gas. I thought it might have been from the rain. But, when we got home and I cleaned out the carb, the problem was evident... a bunch of dirt and crap from the gas tank of the station we filled up at... an off brand that happened to be there when we needed fuel.
I've also got an '86 Honda Rebel 450 and a Suzuki M109R. All run happily on 87 octane but I'll be sticking to good quality brand-name gas from here on... and not when a tanker is filling the station's tanks. When in doubt, I'll run 89, but for the most part, I get fine performance from 87.