Personally, I started with a CLP. Then I found Gun Butter. And let's face it, AR's tend to run better when kept wet.
And what I've found with using Gun Butter is it not only keeps it wet, it doesn't collect dirt, dust, powder residue or any other gunk that can muck up your AR.
Granted, there are several other great products out that probably do just as well.
As far as motor oil, I'd only use it if it was all I had and I had no other choice. Does it lubricate, well duh, sure it does. It's been lubricating car engines and other things for years. But it also collects does and if you've ever seen what it can do to the inside of a motor over time, well I don't want it doing that to the inside of my AR. If I had a choice, at that time of having no other choice, meaning S has hit the fan, and money was no more, going to a store was out of the question etc., etc., I'd use transmission fluid before I used motor oil.
But I stock up on my favorite lubricates and degreasers when I have the extra cash to do so.
Here's a link to Gun Butter's website. If the video of Todd Jarret is still there of him running 1000rds. in an hour I think it was, out of a new Para 1911 in .45acp with only a few drops of Gun Butter, watch it.
http://www.gunbutter.com/index.htmI've also included a few links of other lubricants I'd use and use long before motor oil and transmission fluid.
http://www.magnalube.com/ <<< the grease is great for M1 Garands and other similar firearms that require a grease.
http://www.topduckproducts.com/products.aspx?Product=GunzillaInfo on GunZilla >>>
http://www.topduckproducts.com/There's a few others but they're bookmarked on the desktop and I'm on the laptop at the moment.
I'm sure some may have an issue with the product I use or the product/s I've suggested. But the one is what I use and have found to work great for me. The others I haven't tried but would certainly do so long before motor oil. Although I hear motor oil works great in AK's. But I also know AK's unlike AR's have very loose tolerances and the thickness or viscosity of motor oil fills the gaps. lol
Hell, I bet and AK would even run great with some 90 weight gear oil? I don;t think it would smell all that good after it heated it up because 90 weight gear oil tends to stinks as it is. lmao