I have a Brison 3.2 that would lean out for no reason when in flight after I started to run synthetic oil after break in. Had the same problem bringing here in dead stick. Wheel pants caught on the high grass ripped off gear broke cowl and engine mount. So I went back to the sthil chainsaw oil non-synthetic 50/1 mix after I fixed the plane and motor has ran great ever since. It does sputter a little in the mid range on the 50/1 but never quits and the plug looks great so Iam sticking with it. Like I said not sure why but it seem this engine runs better on non-synthetic.