Almost any lubricant that can provide some friction reduction and handle the high heat and pressure that the ar-15 BCG takes will work.
I agree with the others that say motor oil is a marginal gun lube. If you are going to use a motor oil, I prefer a thicker, heavier oil that stays in place. 15w-40 or 20w-50 HDEO is preferred for thickness and better additive package compared to a normal automotive oil.
My favorite lube is corrosionX. It clings well, reduces friction, lasts a long time, is an excellent rust preventer and can be used to clean. It is basically non toxic, has a very mild odor and is cheaper than a lot of gun specific lubricants out there. I highly recommend corrosionX!