Too much lube is bad (it can migrate into the chamber, can create "hydraulic" effects in cavities & recessess, can become burned into "lacquers", etc.).
Too little lube is bad (it can cause premature wear, seizing/galling, rust formation, etc.).
In normal enviroments, and with reasonable cleaning techniques, "crud" will not cause malfunctions. AR's short stroke for all kinds of reasons, including too much or too little lubrication.
So, how to do it?
The AR15 is a filthy rifle! It's the only design I know that actually shuttles all the debris from the bore directly back into the bolt/bolt carrier! So, first, you must keep this rifle clean; after every shooting session! Squeaky clean!
Then, spray all parts with a teflon based light oil (Tetra Gun spray is all we use). This will protect all working parts from corrosion. Wipe off all excess!
Finally, use a paint brush (we use acid brushes, cheap & available at all hardware stores) and paint a "film" on all working surfaces (bolt body including the lugs, bolt carrier, cam pin & cam, upper receiver surfaces, and charging handle). The "film" will not migrate, and your rifle will thank you! CAUTION! Do NOT put any grease on the barrel extension, chamber, bore, or the firing recessess of the bolt!
Oh, and put "too much" in the buffer spring, buffer & tube. It'll sound so much better, and your aluminum buffer tube will last a lot longer with that hardened steel spring grating back & forth with every shot!
Just my opinion.
--Clint McKee