Foam I would say is messy as it's sticky and has a visual effect. Oil and some slime, especially thin and/or uncoloured, appears closer to wetlook.
Shampoo itself I would say is a gunge as it's made with very similar ingredients (xyz-cellulose) BUT isn't messy as it's used for cleaning! In the same way someone putting on a face mask as a beauty product is not WAM in my mind. It's also again not generally coloured so you don't see the effect.
I put it as 'somewhere in between' as I'm too indecisive to choose.
Shampooing (with soap) is one of the common obsessions that (I've observed) tend to coexist as "sub-kinks" with WAM along with other examples like bare feet, playing with long hair, smoking etc.
It is highly relevant and appropriate, though, as, although it's literally a cleaning agent being used in a sensualised, performative process of cleaning, it creates a foam. Foams like shaving foam are widely accepted as WAM (whether many of us like it or not!)
Conventional soap shampooing doesn't do a lot for me, personally, as the entire focus of a piece of media and neither does any kind of foam.
(Unless, of course, there's a contextual angle like, say, it's carried out on a fully clothed volunteer in the street wearing an unprotected suit. Then it's matter sufficiently out of context to create the necessary anarchic chaos.)
Its conditional, no pun intended. A shampoo can be done in a way that activates the wam neural pathways, especially if the shampooing is done messily, with facial coverage or using a non shampoo substance.
of the two its more in the wet than messy for me because it can be completely washed of......eventuly, without using anything else but water saying that its not wetlook due to the foamy / slimey look depending how much you use and weither its neat or watered down
easier to sub divide messy and wetlook to make cross over substances fit
i've tried washing off shampoo / bubble bath on a clothed walk but it takes forever to wash it out with a hose or even jumping in a pool a few times, it just keeps foaming up on the way home
interestingly if you mix it 50/50 with custard it stops it foaming up but it feels nice and foamy due to the aeration it gives the custard on a custard walk