z wrote

WD40 is thin to begin with, and at least evaporates leaving no
gunk behind,
So you want something light/thin, that evaporates without leaving
a residue, but not too quickly. Maybe dunk it in kerosene?

Despite the manufacturers claims, WD40 is for the most part
just kerosene with perfume added.

That being said, I'd suggest WD40 for padlocks. Any lubricant
will eventually trap dust and other particles but at least with a
spray can of WD40, you can blast any crud out of the padlock.

If the OP is really concerned about crud getting into the padlock,
get one of those plastic covered padlocks with a flap that snaps
over the keyhole; http://tinyurl.com/kqalr