Dan, this line from your email caught my attention:
On Mon, Sep 19, 2016 at 10:56 AM, Dan--- via Mercedes <email@example.com
> The only reason for using the higher viscosity oil is to keep idle
> pressures up in high temperatures. In both M119s with over 275k the idle
> pressure drops perilously close to 1.0 bar during the hottest times of the
> year with everything turned on. If I use the 15W-50 idle pressures are
> more like 1.5 bar under similar conditions.
My understanding after reading through the "Motor Oil University" material
on Bob is the Oil Guy ( https://bobistheoilguy.com/motor-oil-101/ ) is that
engine oil is serving two purposes, lubrication AND cooling, and in order
to cool it needs to flow, at the rate of flow that the MB engineers
calculated. Increasing the viscosity will slow down the flow (less
cooling). You may be using the wrong metric (hot idle oil pressure) to
guide your choice in oil.
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