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Recently purchased new Bridgestone tyres (same as originals) for my Suzuki Grand Vitara - noticed bright green valve caps and was told that Firestone always fill with nitrogen gas - nobody asked me but note charge of $5 each tyre included !
Dealer states that it is Ok to top up with air so what is the advantage to me ?

This is upselling and if you didn't give your approval for the additional charge, the dealer really had no right to do it in the first place.
In theory, nitrogen filled tyres retain their pressure longer, so there should be less need to top up. However, if you do, you can top upwith normal air (after all, it's 78% nitorgen anyway!).