They state so on the bottle. I would assume it leaves a staining thats visible.
This is in their FAQ on this link: Wheel Protector
Q: What type of wheels is Wheel Protector suitable for?
A: Wheel Protector is suitable to use on Split Rims, Diamond Cut, Refurbished, Multispoke, Anodised and unlacquered wheels. Wheel Protector is not suitable for any wheels that are Black, dark in colour or have a mirrored finish.
Q: Why is Wheel Protector not suitable for black, dark or mirror finished wheels?
A: The finish will not be acceptable. It will leave an uneven oily looking film. We would recommend that you could use High Definition Wax, Extra Gloss Protection, Super Resin Polish or Instant Show Shine, they do not have the super hydrophobic quality of Wheel Protector but would protect them and make future cleans easier.

