I heard the same argument on the changing of the fluid, I will let the wise one's debate this one out, personally I would change it. Couple of things to remember, Dip stick is not in the tubing and you have to buy from MB, it is sold as a tool. Also, use MB Fluid ONLY.
Make sure to buy the red tab to seal up the dipstick tubing .20 cents.
Oil is $10 a quart/
Your fluctuation is speed is probably caused by the Blower motor regulator, I dont know if you can buy that as a seperate unit or have to update the blower motor as well, check with your dealer.