Obviously, wipe it out and try to squeeze the excess oil out of the air filter. Also, since you lost some oil, make sure it isn't too low before firing it up. The bike has to be level to read the oil, you can't do it while on the kickstand.

 

The hotter engine oil gets the more vapour it produces, when that oil/air vapour mixture hits the airbox it cools rapidly and any oil vapour quickly condenses and non-condesable gasses are sucked into the intake tract.
The maintence schedule is very lean and should be only used as a guide. 10000km oil changes, yah right. I got 3000km on my KLX and have done at least 6 changes, 4 filters. I'd recommend to clean the air box out really good, get a new pre oiled filter, I don't recommend a KN filter they are too porous. Run good oil and change a lot more frequently than suggested in the service manual, especially if you are riding hiwsys or riding hard off road.

 

what kind of oil should we rub to the air filter after the cleaning?
last year, I cleaned the filter and it was so messy all soaking wet. now I suffer from some starting problems maybe that was my problem in the first place.

 

I use a spray oil made for air filters. Spay it on generously, then work it in squeezing and kneading the filter. If you do not want to get your hands oily you can put the filter in a plastic bag and do the squeezing and kneading in the bag. Then wring out as much oil as you can and install the filter. I also grease the sealing surface to make sure it seals. A little smear of grease around the base on the filter will do the job.