Your engine is liquid cooled right? I'm assuming it's an automatic so you will have not potential wet clutch issues with friction modifiers.
Don't sweat the recommendation for motorcycle oil. That's B.$. I've owned and operated Suzuki air and liquid cooled engined ATVs for twenty six years and never had an oil related problem and I've NEVER used a motorcycle specific oil. I'd say Suzuki engines are as good as anything out there and don't require any special lubrication although, proper viscosity oil should be used.
I presently own and operate two liquid cooled 500cc ATVs. One auto and one manual. As with all of my motorcycle/ATV engines, I run HDEO (Shell Rotella 15W40) conventional oil. Not sure what kind of riding you will be doing but if you're going to be punishing the machine with lots of high revs going through deep water, mud and dust, you'll be better off with HDEO.
If you're going to be taking it easy on the machine (normal ATV riding), I wouldn't be too concerned using either 5W30 or 10W30 at least for your initial short term oil changes and wouldn't be hesitant to use it in cooler temps.
What I find unusual, is that my 500s each have different oil recommendations. My 2006 auto recommends 10W40 but my 2007 manual recommends 10W30?? Go figure. Not that it matters to me. I run 15W40 unless running them in the winter. Then I use 5W30.
The most important thing about lubrication used in ATVs is frequent oil changes. The nastier the conditions, the harder the engine is pushed, the more frequent the OCI should be. I used to do mine every 500 miles. I've since changed that to 1000 miles because the oil was coming out almost as clean as it went in. These machines aren't seeing the hard service that my old air cooled Suzukis did.
If you plan on doing year round riding, you might want to use HDEO during hot weather and switch to the lighter oil during cooler temperatures. That's what I'd do.
A word of warning.... Keep an eye on your CV joint boots. If you're riding in recently cut woodlands with lots of stubble and sticks protruding out of the ground, your chances of tearing a boot are pretty good. It happened to me. And, I'll tell you, replacing a boot is a lot of work and a bit complicated for the uninitiated. If you bring it in to a shop, there will be a hefty bill for your reward.