As has been said, PB is for breaking stuff loose. My experience differs from Bill's in that I have had better success with it that Kroil, but I don't wanna start a mechanic's version of a caliber war here...
Anyway, to those who use motor oil: How does it perform for repelling dirt? I have just started using Slip 2000 EWL on my revolver and am impressed that it doesn't pick up environmental crud like Rem Oil loves to do.
And why 5W? It seems that something a little heavier would be better for guns, as its role is more as a light grease than a flowing lubricant.