Air filter maintenance is something you don't want to save money on. The filter is important but more important is the oil you put on the filter. It is the oil that is stopping the fine dust particles from reaching your cylinder. The filter foam is just a way to suspend the oil in the air. Use only foam filter oil. Motor oils and others do not have the sticky properties required to stay in the foam. I have used Bel-ray(is thick from the bottle and requires more to get it spread through the filter. Right now I have been using Silkolene, which is thinner and spreads easier through the foam. I oil the filter by placing in a ziploc bag, pour in some oil, smash but don't twist the filter so the oil is pushed into all of the foam. Depending on how skilled you are you may need to squeeze out the excess oil becuase too much is a waste and also makes a rich fuel mixture becuase the engine is not getting the full air flow it needs. Your 90 kdx can use twin-air and uni filters. I like using a uni filter(one coarse foam and one fine celled foam) when it is muddy out because lots of junk can get inside the airbox if you run without the lid(as most do). The coarse foam catches the big particles of mud and in my mind allows more space for the air to pass through the filter. Lastly, I use to use the soap and water cleaning method but after I tried the cleaning solvents for air filters I was hooked. It takes no time to clean a filter and the solution is easily rinsed out of the filter with a garden hose on slow-flow. Oh, I forgot, I'd bet that your bike still has the stock filter and the foam could easily be past its life. Just trash that baby and get at least two aftermarket filters so you always have one ready to go in a clean ziploc bag. One days riding on a dusty coarse can fill the filter completely. Some people change filters twice a day depending on the length of the ride and dust conditions.