Originally Posted by LantanaTX
Engines are built with certain clearances and the wight of the oil need to conform to those clearances. Oil acts like little ball bearings between the surfaces. As an engine get older these clearances start to increase and so you can extend the life of your engines by increasing the weigth. My engine has 61,000 miles on it and loves the 5w 40 Mobil One. 5w works great in cold weather, so since MB reccomends 0w I am assuming these engines are built with very tight clearances. If my engine still only had 7,000 miles I would still be using the 0w.
I think that LantanTX puts it right. Its not what we think we should use because oil is oil but rather how the engine is designed. The oil needs to lube the engine. That is its primay reason for beign in there and if it cannot get into all the spaces that it needs to lube (piston sidewalls and cylinder) you will have issues so too heavy an oil not enough lubrication and cooling. THe viscosity that we have heard about refers in laymans terms as "resistance to flow" and the higher the viscosity the higher the resistance to flow.
Shear and Time are also two elements that are considered when determing viscosity, and withoug getting into detail on these terms, simply know that oils have ratings for the precise reason that they differ for varied applications. THere is a bit of science behind the oil refining process. Oops did I just say that? Maybe thats why the world is fighting over it so much.. Sorry another discussion for another day
MB knows this engine and what works best in it. They do not want more warranty claims than they can shake a stick at so they tell you what they recommend. You engine is designed to precise tolerances and while there is definitely some latitude built into these designs, your car will work best following the engine oil recommended.