I'd go with the "severe" service schedule in your owner's manual. The "normal" schedule generally allows for far too long of mileage intervals for basic maintenance.
The trannies are so-called "lifetime fill", which I interpret as the life of the tranny, not the life of the vehicle. Probably one of the stupidest things that new car manufacturers have done. They've eliminated the dipstick, so the fluid needs to be pumped in thru a special fitting after removing a plug on the bottom of the tranny pan.
It can be done by a do-it-yourselfer, and there's plenty of information on how to do it in the transmissions forum. Just make sure you use genuine Mercon V fluid or suitable equivalent. Regular old Dexron/Mercon will cause shifting issues and/or premature failure.
Your tranny is a 5R55 series. There were a few suffixes after that denote the gearing, etc. but they're basically the same.
