Not many people know that TwinAir filters are actually made by/contracted by Hiflo Filtro -- it's the same thing with the TwinAir oil filters and the K&N oil filters, both made by Hiflo. The only difference with the TwinAir air filters is that they have an orange color on the filter; the Hiflo has a green color. Oh, the most important part, the Hiflo's are about 1/2 the price of a TwinAir. Best filters ever. The part # is HFF2019 for an 87-03 KX500 filter, you can find them for less than $15 on Amazon. I usually just buy 2 or 3 of them at a time and rotate them. I get really long life out of the ones I use in my 2009 KX500AF, 2012 RMZ450, and 2013 KTM 250SX. I even clean them with gasoline, but I do blow them dry with an air compressor. If your grommet works, I'd just stick with it. Never a bad idea to apply grease also.