I expect that there is a problem with the translation. Benzene is less harmful than thinner, but indeed it is a "harmful" product. Therefore, only outside use.
Sorry to insist but why use "heavy duty" mineral based degreasers on the sidewalls of a tubular?
Most of the tan walled tubulars I'm familiar with are coated in latex to prevent them from soaking up water. Using mineral spirits on such a coating will slowly but surely destroy it allowing dirt to actually penetrate the sidewall making it irreversibly "dirty" looking.
Benzene (benzeen in Dutch) is not benzine (gasoil, benzina) and is mostly used in pharmaceuticals, they are chemically related though and indeed, both are toxic. I'm actually rather surprised to see it's commercially available.
So far I've always cleaned the sidewalls with a mild detergent mixed in water and a spunge. Works fine for me.
The achilles heel of tanned sidewalls is the rear wheel where you need to take care no soiled cleaning fluid from the cogset is dripping onto the tyre's sidewall where it will penetrate the casing as I described above.
I take it the FMB tubs do not come with a protective coating?
Either way, I'd still advise against such strong degreasers as they'll likely enter the layer of rim cement where it will weaken the bond eventually.