Polaris doesn't use a wet clutch. It is belt drive. Rotella may be a fine oil and I have used it in motorcycles with good results, but Polaris calls for a 0W40 which is a little different than 15W40. Depending on how much you drive you are talking a difference of $10-20 a year between synthetic and Rotella (2 quart capacity). If you can't afford that, then you shouldn't have bought the 4-wheeler in the first place. If you are really pinching pennies, I'm sure it will not hurt the engine, but a 0W40 should flow and get to the top-end quicker than 15W40, especially in cold weather.