I'd be cautious about that. There are sprays made for cleaning volume control potentiometers, but WD40 isn't one of them. Check out Radio Shack for stuff like this.I posted about this problem last week. Someone responded to pull off the face and spray the volume control with WD 40. You have to unscrew the circuit board first . It works.
Also, I'm pretty sure that my double DIN Monsoon system does NOT use a typical potentiometer volume control. Instead it uses some kind of encoder, maybe optical. I know this for two reasons: 1) there is no stop on the control (I can turn it either way forever and never encounter a stop), and 2) it has indents as it is turned. I've watched it carefully and the volume is controlled in small steps, rather than continuously.
Because the volume level is controlled by an encoder, some sort of microprocessor is used to handle the encoder output and convert this to a volume level. That, in turn, means that there's something about that microprocessor that prevents the volume from going low enough. Fix the uP and you fix the problem. That fix may be as simple as a master reset, however that may be accomplished. Of course, it also could be a problem someplace else.
Note that the tone, balance, and fader controls are analog potentiometers. I'm sure that was done because those controls don't get the use that a volume control does. Because of the way potentiometers are made they will eventually wear out, but an optical encoder won't.