Sounds like your one way bearing. Put the machine in reverse and try to push it backwards, if it rolls it's the bearing.
Yes we can run synthetic, Cat sells kits that come with oil and filter that are synthetic, I've been running their 0W40 synthetic for quite a few years in different machines. Just have to be sure it's wet clutch compatible.
2010 Prowler XTZ 1000-
27" Grim Reaper Tires/Home Made Roof/Arctic Cat Mirror/Quadboss Folding Windshield w/ home made support bar/LED Tail-Brake lights//Venom 4000 lb Winch/Amsteel synthetic line/Home Made Doors/Custom Adjustable Seats/Custom Oh Crap bar for passenger/Tonneau Cover/Fat Max storage Box in bed
Tiger Tail Towing system