the reason you need to check the fluid level when the vehicle is in gear OR neutral is because when either in gear, or in neutral, it will pump fluid. i just prefer in neutral cuz its safer, for obvious reasons. when the tranny is in park, it isnt pumping fluid, its just sitting there. so the fluid level will be different because its not being pulled/pushed into the lines, the cooler, the transmission itself, and in/out of the TC, etc.. this is the reason that if your tranny is starting to overheat, and you pull over and park it, the temp will continue to rise. because while in park, you arent pumping fluid to cool it down. when overheating, you are suppossed to pull over, put it in nuetral, and let it high idle. it will cool your tranny because it is putting fluid through the cooler without any load on it.