Fourwheeler sittin for a few years

So, I have been handed down a 05 Rancher ES 350.

It has been sitting for the past 5-6 Years, and hasn't really been ridden besides once a year. It sits on my property, and no one touches it (as far as I know). I really wanna rebuild it, and upgrade it to maybe a 400. The only thing is, it gets a oil change once every 5 years (from what I'm hearing). It runs pretty well as I see, but I only have had it for 3ish months. I don't know if I should stop where I'm at, and rebuild the engine now or what. I just am looking for what to do from here on.

Thanks for the read. :big-grin:big-grin