Okay, lets see.... lubrication. Generally speaking, you won't damage the rifle by over lub-ing it, however, the AR is a gas-operated weapon. If there is excessive grease/oil in it, more junk will tend to stick to it. It will make cleaning it a lot harder, and if left to cake up on parts within the receiver, will lead to reliability problems. Most Mil-Spec AR's are finished in a mil-spec teflon parkerized type finish and are fairly resistant to rust and corrosion as is. Your best bet is to put a drop of CLP on each of the moving parts in the lower, a drop on the bolt where it meets the carrier, and a half-a-drop on the extractor. Most new barrels as well as A-2 milspec barrels are chrome-lined, so a light coat of CLP for storage is all that is needed. As far as the 30 round mags being unreliable, that came from the early M-16 during Vietnam. The army had problems with them due to weight and stuck with the 20 rounders. Since then the design has been improved and with an A-2 spec rifle and current style 30 round mag, you should have no problems. I use 30 rounders exclusively in both of my bushmasters and I've yet to see a jam or failure to fire at all. Make sure to do it right, and use quality parts, and it will serve you well. They are fun to shoot, and the platform is highly addictive. Good luck.
-Spyderman