You landed right in the middle of the best expertise on this platform you could possibly find on the web! Welcome to the forums! Your rifle is in fact a .308 rifle. It is chambered with 7.62x51 reamer, which means that the chamber is designed for NATO spec 7.62x51 ammunition. The technical explanation being quite lengthy, your short answer is that the .308 Winchester and 7.62x51, are externally the exact same round. Internally, the difference is in the case thickness and SAAMI specs, the primary difference being that the NATO round has a thicker case. They are interchangeable. Your answer will depend on your primary use for this weapon, if you need the most you can get accuracy wise, .308 Winchester match ammo will probably give you the best results. If you want to shoot as affordably as possible, 7.62 military surplus or equivalent will probably be your go to option. We're proud to have Armalite as a forum sponsor, you can simply contact them through their website, they should be able to tell you exactly what you have based on a description and serial number. I'm sure they'd be able to answer all your questions with a simple phone call. In the collective experience here, these larger frame ARs like to be run quite well oiled for their first couple hundred rounds or so. Sometimes it's necessary, sometimes it isn't but you have nothing to lose by being prepared to oil it up if you experience any problems within your first few rounds. Enjoy your new rifle, and again, welcome to the forums!