will going up a viscosity grade do any harm? Because the way I see it it will prolong engine life.
I see it the same way.
But as usual - it depends.
If you drive a lot of short distances, for city cruising, or if the engine often doesn't reach operating temperature, etc., a thinner oil might be a better choice. However, for long distances, high loads, towing, and in gernal higher operating temperatures, a higher viscosity provides a more stable oil film everywhere, I think.
The problem is that modern engines are optimized for lower fuel consumption and therefore take much longer to warm up. My 23 year old C220 TDI engine warms up sufficiently within 10 km at average ambient temperature, say around 20°C - which will most likely not be the case with a modern diesel engine.
I intend not to start the Grenadier for short distances. For that I take my scooter, my diesel motorcycle or the SLK. The SLK's coolant reaches operating temperature in less than 5 km and the heater already works after only 2 km, even in winter. This is not possible with a diesel, in particular not with a modern one and for sure not with one as big as the B57.
Different engine generations, different engine types (diesel/gasoline) and different driving profiles require different oils.
I certainly won't be using [0/5]W-20. But I will try to find the right one for my conditions, which will as a learning process take some time to know how the engine temperature develops in my use cases. For me the question is whether it will be [0/5]W30 or [0/5]W40.
If I know I have a longer trip coming up, such as to Sicily or Sardinia, the Pyrenees or Greece, it might always be worth switching to [0/5]W-40 at least for that time of the year.
EDIT: [0/5] indicates that I'm not yet sure about a 0W or 5W specification.
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