The link that you posted is for the ZF 5HP24 transmission used in the 4.4i & 4.6is V8 E53. Your 3.0i is fitted with the GM 5L40-E transmission. The Pentosin ATF is a replacement for the Mobil ATF LT71141 fluid used in the ZF transmission (golden colour). Your transmission uses Dexron fluid (red colour). It's essential that you don't mix up the two fluids.

How certain are you that the plug you're removing is the level/filler plug and not (say) the pressure test plug? Could you maybe take a photo of it and post it up on here?

Phil