Here's a good question. Why is 10W40 so bad? I know people say that since it has a larger spread than a 10W30 (30 points instead of 20) that it can break down a lot faster and cause sludge, ect... But my arguement to that would be this: almost all Honda, Kawasaki, and Suzuki motorcycles recommend 10W40 motor oil for their motorcycle engines. Now remember that an oil for a motorcycle must lubricate BOTH the engine and transmission, operate in higher temperatures, operate in a higher RPM range, and most are air cooled. Also, like my brothers Honda, the motorcycle has no oil filter. On top of all this the manual for the Honda recommends changing the oil every 4 months or 4000 miles. It seems to me that oil for motorcycles would get a pretty good workout. Plus no oil filter and a longer OCI and you get the picture. I just can't believe the bad claims of a 10W40 if it has to be used in these conditions without breaking down. What do you guys think? Oh on a side note I have another good question. My car manual says you can use 5W30 from 32F and anything below (for really cold weather as in -10F, -15F, ect...) It recommends not using the 5W30 above the 32F mark. It also states you can use 10W30 from 0F - 100F+. My question is why can't you use 5W30 when its 80F outside? They are both 30wt. oils when at operating temp. It seems to me the 5W30 would only be icing on the cake - better flow on start up. Is it because the 5W30 can shear down more than a 10W30? Someone please comment on this. Thanks everyone.