I have tried various filters, but have found the stock filter to work great. As Manny and Paul can attest, some of the Nationals are pretty darn dusty, especially in Nevada. I use PJ1 spray on filter oil, and spray on both the inside and the outside. I then toss the filter in a plastic grocery bag and squish the filter to make sure the oil is spread throughout the filter. I have NEVER had a motor failure caused by the filter, even in races so dusty that the bike would barely run at the end of the event. In one really dusty Nevada race a few years ago, they must have had 20-30 riders that lost motors due to dust getting through the filter. On all three bikes we had in the race (all with stock filters and PJ1 spray on oil), none had a problem.
For cleaning, I use a parts washer with solvent, squeeze (don't wring) the solvent out, rinse the solvent in hot soapy water, and then rinse again in clean hot water. Squeeze dry, and put on the shelve for air drying. I have six filters, so only clean filters every couple of months. I have heard that using solvent will ruin the filter, but I have filters I have had around since 1999, and they are still in perfect condition.
Just my $.02...
Rick