I had the same issue with my Super 8 150. Dealer said it was normal to get a little gear lube in the air box, even if it's filled to the proper level.
What I did was put a small piece of foam sponge in the air box, in front of where the breather tube comes in. Any oil that does come through the tube gets caught by the sponge, and doesn't spray all over the air box and filter.
At my last oil change, I had no oil on the inside of the airbox to clean up. Just a little in the sponge. I just washed the sponge with warm water and dishwashing soap, and dried it out with a hair dryer. Put it back in, and good to go!
And no more oil in the air filter clogging it up!