Viscous fan - the cooling fan behind the rad - not the electrical aux fans. The viscous fan changes speed with temperature - spinning slowly when not required and speeding up as heat requires it to. If bad will not speed up all thew way causing higher engine temps.
Mercedes Antifreeze is designed not to attack plastic or hoses - read article from MBUSA - NA antifreeze deteriorates plastic (hence rad neck breaks)Using MB antifreeze - according to the article allows hoses to last up to 15 -20 years. Some disagree with this - some agree - you can draw your own conclusions - I am switching to MB antifreeze on my 560sel when it comes out of winter storage.