I have been running the cheap Castrol Oil in my sleds for over 20 years. Including the 800 Revs. Owned an '05, '06, '07. also have had the 550f, and 600 HO. My '06 has the SLP 163 hp kit and I've run the sled for three years now without any oil related problems.
In all my years of running sleds (and I'm NO trail rider) I have never had an engine failure.
Once a year you might want to pull the exhaust valves and clean them up a little. But I've never seen anything bad enough that would hinder the performance of the valve moving up and down. just my .02 worth.
AND I have heard it's hard to get an engine (rings to seal) to break in properly with synthetic oil. Petroleum oil helps the rings seal quicker (better compression). We use to run our motocross bikes with a heavier mix of oil/gas, because the heavier oil (mix) created better compression