Unless it states "Full Synthetic" it is a cost saving compromise. Read this Synthic blends, make it yourself.

 

I have seen the bubbles you are talking about on other people's z400s. But in both cases they were running a synthetic blend......

I've been running Amsoil in mine since the first oil change and I've had no such issues.......... I'm sure there are other options out there that are just as good but I've always run Amsoil in every ATV we own and to this point it has performed perfectly in all temps and conditions.....