I think that Thymeclock’s post is pretty much the definitive answer on the subject, but I’ll just throw out this bit of info that NGK presented at a seminar they held for all the local shops here. I don’t know what the U.S. laws are on fuel labeling, but here the warning “CAUTION. MAY CONTAIN ETHANOL” is all that is required on blended fuels, so the percentage of ethanol going into your tank is unknown. According to the NGK rep, the percentage here could be as high as 20%. They suggested that with that much ethanol a colder heat range plug is required, especially in heavy vehicles that work hard. They also said that the key to success with colder plugs is to tighten the gap 5 – 10 thou. Our shop has made the change to colder plugs for lots of customers with feedback from only one fellow – he said his big block equipped Chevy truck pulls his trailer up the grade somewhat better but he has lost 2mpg (Imperial gallon). So any further thoughts on heat ranges and plug gaps for our little engines?