250-500cc Oil in airbox

Since I installed a Keihin in '09 WR300 I notice there's a fair amount of oil pooled in the bottom of the airbox whenever i clean filter. Definitely not filter oil - use water soluble stuff. Looks & smells like 2stroke oil. Don't know how it gets there,but when I brake into a corner then try to accelerate after cruising in top gear a while ,it almost dies & takes a few seconds to clear again.I believe the oil is running down into carbi.
The jetting is nice & crisp- 172M,JD red needle/middle clip,48PJ, 1-1/2 turns a/s. I set the float lower too without any effect.
I'm completely puzzled, any ideas?

Crankcase oil?

It's just premix oil. It's normal. All of my 2 strokes do it. I just wipe it out whenever I change the air filter. I doubt it effects how the bike runs.

+1 on the premix oil. My 2010 WR300 does the same thing and no running problems at all.

Thanks for replies. So fuel must blow back in the airbox,evaporate & leave oil? Theres quite a bit of oil in there,enough to run into the carb under braking.Its the only explanation I can think for the massive bogging thats happening sometimes. The jettings spot on,tried diff plugs,lowered the float & run at 40:1.
Should clarify,oil is pooling in the boot between filter & carb.

What do you mean by water soluble stuff, are you not using standard air filter oil ?