I used Windex without any problem on my granite island. About every 6
months, after cleaning it good, I applied some wax-like polish for
marble/granite, which brought the gloss back up just fine.
Don' use Windex or anything with Ammonia in it. That will remove the sealer.
We use GLASS PLUS, although it looks like Windex (blue) it has no ammonia
and leaves the granite clean & shiny