There is a "sweet" spot for each and every diesel (as well as
gasoline) engine [depending on Design, Use and MAINTENENCE.]
with reference to operating temperature.

Barring direct communication from: The Almighty , or Rudolph
Himself ; I would default to the Engineers @ M.B.

Throwing out any "manipulations" for our silly non-diesel
oriented emissions, the Engineers have designed the
optimum temperature range.