[MOTORHEAD]

Have been using a tire dressing to keep the tires looking new (low sheen, not shiney ) but recently I've noticed a brownish coating on them that does not come off easily, but it will come off if I rub it with my finger or a cloth, but in ribbed or lettering areas it is difficult to remove. I'm thinking it must be the tire dressing causing this (yellowing) .
Don't know the chemical make-up of what I'm using, but must be silicone or ???.
Anybody have similar experience with this problem?
If not, what are you using that doesn't give you any problems?
I'll post some pics later.