car oil is not good on the clutches and wont last as long in an atv engine as atv oil. Valvoline makes an awsome atv oil. Also there should be a clutch adjustment on it.
For play the 420 AT Rancher. For work, build up and mud the 550. Also consider the 700 Grizz. I think it is $600 to $800 more than the 550. Both are proven quads. Just depends on power and what fits you.