Don't know about yours, but the owner's and shop manual for my 2005 Sportsman 500 HO calls for every 1000 miles or 100 hours using Polaris's 0-40wt. Synthetic. I always use it for the reasons I posted on the Yamalube oil thread about 6-7 threads down.
As far as the air filter goes, that totally depends on how much dusty riding conditions you subject it to. I bought my Sportsman about a year ago and was surprised regarding the complex design and engineering they put into ATVs nowdays, meaning it would pay you to get an owner's manual so you service it correctly. For example, if you have the ON DEMAND four wheel drive, it's real important to use the correct front differential fluid rather than any diff. gear lube.