I have a DuraBlue foam dual filter, (same as Banshee). I wash it in petrol, leave it to dry for 20 mins, then I soak it in filter oil. I also put on a filterskin over it "dry" to catch all items over 0.005micron.
I must say, they have never found a spec of dirt inside my carb at service time. I also clean the filter after every 10 hours of riding.
I am told the reason the manual says no gasoline, is to prevent an accident and Yamaha being sued. It works well and evaporates quickly, which is important before you oil it!