I was told that ordinary glass cleaner (Windex) would dull granite. Is
this true? I have a product that was given to me when I got the counter,
and its almost gone. Was wondering if I have to re order some of it or I
could get something at the local HD.

Very few things hurt granite, even kitchen cleanser has not dulled mine.
windex is mild and wont hurt it.

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

R.