... After a year or two, it turns into some sort of sludge-like varnish that is almost impossible to remove! I've had to soak guns for a week in solvent to loosen the WD-40 before I could even disassemble them!
IMO, it fails miserably as a lubricant.
I wonder if the WD-40 may have partially dissolved other old oils or cosmoline which redistributed and re-solidified in different places after the WD-40 dried out? I haven't experienced WD-40 itself turning into a "sludge-like varnish". Just interested.
I consider WD-40 to be a penetrant and "temporary" lubricant, as for removing rusty bolts.
I can't say as I bought these used and they were already "protected" using WD-40 according to the seller. I did know what I was getting, and the end result was fine, but it was a PITA to get them apart the first time!
I use WD-40 to flush water off of mechanical things, but truthfully I have had a spray can of it for years, and it's still pretty heavy, not much use.