i've been using synthetic for 2 years. the issue is, it cannot have friction modifiers or they will negatively affect the way the clutch grabs, since the tranny is using the same oil. i find that the engine runs much cooler with the synthetic oil - my fan hardly kicks on anymore. it used to run most of the time, since it's mostly mountains here. i use Shell Rotella T 5W40, but i think there are other brands (amsoil, etc.) that will work, also.
the other issue with going from conventional oil to a synthetic is you must have done scheduled oil changes and not allowed the engine to have built up excessive sludge. i used to work for mobil oil when mobil 1 was one of the first synthetics on the market. we would get complaints of oil leaks once people changed to a synthetic oil. here is the problem: if conventional oils aren't changed regularly and they build up sludge, it will begin to eat through the gasket material. this isn't a problem since it basically seals the leak with the sludge. but, once a synthetic is introduced, it will clean up the sludge left behind and a leak will develop. so, if you haven't changed oil at regular intervals, or if you aren't sure of the maintenance history of a used quad - i wouldn't suggest changing over to a synthetic. but if you have done the oil changes regularly, a synthetic should greatly extend the life of the engine. just be sure to find an oil without friction modifiers or the clutch will slip. i'm not sure about the automatic trannys and how the oil would affect them, though.