Hey Jeremy welcome to the club. To answer your questions in order: purchasing oils and filters from the big box stores you mentioned I know a lot of guys use Rotella or Mobil One 0w-40. As for a filter idk the cross reference for it so hopefully some XP guys chime in. Personally I prefer Polaris brand oils and filters. I generally make my online purchases from https://www.rockymountainatvmc.com. Now for your EPS light that is most likely the sign of a weak battery. I would suggest putting it on a float charger and see if that remedies the situation. These machines have power steering units that are powered by electricity not hydraulics like an automobile, so no fluid needed. As for any additional things to do on the wheeler I would say proper maintenance is critical and that would entail greasing, oil changes (differentials, transmission, and engine), and air filter. I change my motor oil and drive-line fluids every 50hrs. I grease the machine every 3-4 rides ie days of riding. Air filter 2-3 times a year depending on how dry it is and how dusty the rides are. There are also a ton of mods to do but that would require more info from you. Good luck, that 850 is a beast of a machine especially for a first timer.