Shake the keg good and depress the poppet, do the gas one too, let oxidizer sit a few minutes, then repeat w sanitizer. Let it shoot on out a few times. I use a 10" landscape spike, just 'cause I like a convenient tool I can see for each job. None of this has to take much time, just working while wort is boiling, or if running distracted, after whirlpool while trub cone is settling.
For initial keg cleaning or refurbish, soak take 'em all out (the dip tubes that is) and soak well in favorite oxidizer.
Something like this works too, esp for all the dry hoppers or refurbishing old neglected kegs;
Beer Line Cleaning Brush - 3/8 in. x 48 in. | MoreBeer
I for one, would not want gun oil in my beer, or get keg debris in my bores.