I just bought my first atv. A 2005 Polaris sportsman 500 H.O.. It has the same problem that I have read about with the gas in the oil. Suggestions have been to change the oil. This atv did not have a owners manual and I have been unable to download one from polaris. I have also read many threads about oil but I am still unsure about my machine. What type/kind of oil should I buy? Thanks for the help!!