Do not wash your foam filter with anything but dish soap,let it dry and use the same oil you mix the fuel with to oil your filter squeezing out as much as you can but getting it all throughout.
Yes it works just fine at first but will crumble into dust very soon when using gas.You will have quite a mess rebuilding your carb in short order.

Washing the filter in premix works best if you do it in a hubcap off your old van. It was part of our pre-race ritual in the old days. (For me it was a '66 Ford Econoline with sixty-six empty beer cans in the back). It was the van that would not die.

I always recommend a light to medium coating of regular grease on the inside of the filter box and lid along with two stroke oil on the foam filter squeezed out.  Like said in other post don't oil the foam so much as to cause an overly rich condition, lightly but thouroghly oiled. You will be surprised at how much dirt and debris are trapped by the light grease coating on the inside of the air box.  Yes you do have to clean the air box each time you clean the filter but your Yamaha Engine will be much happier burning cleaner dirt free air with the fuel.