When I was at the honda dealer buying oil, I asked him about the hp4, made for or by honda, and it is a synthetic. The dealer said that you shouldn't use it in an atv, and honda don't recommend using synthetics in their quads. You must remember you are using the motor oil for the trany, as well as the clutch so it might have problems with the shearing action, or be to slippery for the clutch. One thing you shouldn't use in your quad is the auto oil since it is made as slick as possible for the best gas milage, and don't have the shearing action, or the clutch. You could use it in a polaris, to try to offset their $10 oil filters, and the two hub oils, the trany oil, the antifreeze, the gearshift oil, along with all the grease, for the pivot points. can you say pain in the a$$?

 

As far as the Honda dealer telling you that HP-4 Synthetic oil shouldn't be used in your ATV, you may want to show him page 96 in your 400EX owner's manual, then ask where his info comes from.

So far from all the data I have gathered, the "shearing" action is marketing rhetoric. No one has provided even the slightest bit of proof that it can cause any problems. As far as the oil being to "slippery" for a wet clutch, that is not a problem either, wet clutches are obviously designed to run in an environment with lubricants. My clutches get about as rough a treatment as possible, with no problems. If it were truely a problem you would hear a lot of people yelling "DON'T DO IT, it WILL make your clutch silp", instead, you just hear a lot of speculation.

As far as "Motorcycle" oils go, all the independent testing I have seen places motorcycle oils at the bottom of the list as far as oils rank. It's all marketing.

 

Hopefully this will help out a little. I run fully synthetic oil in my '86 Big Red and I have had no problems with it whatsoever. The clutches don't slip like most people try to tell you will happen. I also use an "auto" synthetic which people will tell you to stay away from. If you ask me oil is oil. The way I see it, when you buy Honda oil, you're just paying for the name on the side. I hope this helps out a little.