Looks thick enough to scoop it off with a dull knife or spoon. if you wanna use products, go from sfe to harmfull, depending on what the wheels are made of. You don’t want to damage those. I’d start with water. Soak em overnight in water, chances are it will come off easily after that. If not, you can try alcohol as mentioned in other comments. It looks thick, so it will need time to work in. Petrol or kerosine will work, but check compatibility with your wheels first.