"The detergent and the water are solvents, just not petroleum/oil based solvents."
True enough. I thought it was obvious I was talking about petroleum based solvents. Hell, pure water is a solvent!
"I'd like to hear an example of a petroleum/oil based solvent intended for use on guns damaging the metal finish on a gun when used as the manufacturer intended."
I just re-did a gun where petroleum based solvents made a mess of the finish. And the paint I used would be similarly damaged if exposed to petroleum based solvents.
Many guns now days are NOT blued or browned, but are painted. Some of the best paints (read more expensive) are resistant to petroleum based solvents. But because of their price, like bluing and browning, are not suitable for refinishing a gun that is only worth $150. You dont want to be putting more into a gun than you can get out of it unless its an heirloom.
If you are NOT living on the edge, well then, you are just taking up too much space.