I'm sure it was done because you're supposed to know the fluid temperature to get a valid level measurement! VW also does this, but you can get comprehensive scanner software to use on your laptop for $250; on a Benz you can't even get a shop manual! (Why anyone would get a VW with an automatic is a mystery to me, but I digress.)
What's the mileage on your ML? Ours needed a fluid change at 120K. Fluid wasn't low, just needed changing to get the shifts back to normal. Shifting was sluggish and lots of slippage when it kicked down with the accelerator floored. Shifts fine after fluid replacement. Dipstick, fluid, filter, and gasket are available at
The Benz Bin .
If the fluid's not leaking, your level is probably correct. Some DYI'ers have measured the volume of fluid they drained and replaced with exactly the same amount. I think you're better off estimating the temperature and going by the dipstick. If it's 25ºC out and the car sat overnight, the fluid is probably about 25ºC. Estimating 80ºC is trickier, but after a light drive with the water temperature at 80º, the trans fluid is probably near 80º? Not the "right" way, but worked for me—the proof is in the shifts!
