I was always told no more then 6lbs. Any more could blow out the seals. I generally ran with 4lbs till I changed to Progressive Springs,,,, now I don't need to add air .
Also,,,,,,, when adding air to the forks,,,,,,, you need to use a source of LOW pressure, so as not to over pressirize. I always used a portable air tank with no more then 10lbs pressure in it.And then it only took a "quick spert" to bring the forks up a little over 6lbs.Then I would release the air till I got 4lbs.