Mojo,
I just concluded my first season on a Ski Doo 4 Stroke. It's not the ACE but comparable. Your seasonal mileage is well below the average and one oil change per season should be adequate. Most recommend it in the spring at the end of the season to flush out the impurities and coat the engine with clean oil for the summer months.
To the best of my knowledge, there are two oils to choose from. The Ski Doo Brand and Mobil 1. Both would be Synthetic and 0/40 wt. The Mobil 1 is around $7 per quart right now.
I saw an ACE in the showroom, but none on the trails this past season. It's a quiet engine with really nice mileage at around 60 h.p. That'd be comparable to a 440 to 500 cc 2 Stroke for performance, just a bit heavier and with a longer lifespan.