[Dr Stig 2]

Look, don't flame me for asking what may be a stupid question...

Don't you guys salt your roads in winter?

A Z4 is not going to be good for anything but a very short run on hard packed sand.
A short session on the sand is not going to get you to have crusted salt and a big corrosion problem.

I would be more worried about the continuous impact of salted road slush on my car than trying to pick on the chance it was ever taken to a beach for a photo.

The salted road slush may be why all the cars they showed you were so bad.

We dont get this here at all, of course, as we dont get any snow and ice.

Plenty of Aussie guys use a wheel cleaner, and I have seen no one with rust like that on a car that isnt 15+ years old.
I have been using one on my Z4 witht he same 326M wheels and all I ever get is a small amount of rusted brake dust and a gold tint on my brake disc.
I used on on the entire bike (BMW K1200RS) for 2 years every weekend and never saw any rust that wasn't just the brake dust oxidising.

(Ask anyone with an old 6 series or an e30 3 series will and they will tell you they knew how to make them rust back then)

See attached photo. 2 year old regularly cleaned with wheel cleaner. 326M wheel.