This is a must for BMW owners who have moved away from those awful run flat (run craps) tyres. After all, even BMW is now installing Michelin PSS tyres on their performance models. The goo is a latex solution that washes out if you get a flat. The last thing you want is some gunk that congeals in the tyre and cannot be removed, thus making your tyre useless and raises the need for a new tyre. That is the problem with runflats ... you can drive on them for up to 100 kms, but they cannot be repaired.