You can use any liquid that you want in that FG, I use PB for car cleaning. I did use some windex once to spray windows on the house and it worked like a charm.
Pretty much anything works as Beemerboy said. I actually really like how Megs Shampoo Plus performs, does a good job all around in a very very low concentration. I tried it with a little less than one oz and was able to foam my entire car and wheels (98 chevy malibu).