WD-40 will gum up and will harm SOME wood finishes. In moist areas, WD-40 WILL MILDEW and hurt the finish. (Yes, I know guns should not be stored in moist areas, but my point is that the WD-40 itself is what mildews, not the gun finish.)
Also avoid using Hoppe's #9 on nickel plated guns. It will not harm the nickel, but nickel plating has a copper under plating and Hoppe's attacks copper (which is why it does a fine job of cleaning copper fouling out of bores). If it can get to the copper, like at the muzzle or through a scratch, chip, edge of sideplate, etc., it will cause peeling of the nickel.
Gun Scrubber does a good cleanup job, but removes ALL protection from the metal, so if you use it, restore the oil or whatever after use.
My vote goes to G96 Gun Treatment, which has never given me any problems and which cleans and protects.
P.S. W A R N I N G ------------
KEEP ALL THAT STUFF AWAY FROM PRIMERS AND LOADED AMMUNITION
[This message has been edited by Jim Keenan (edited March 29, 1999).]