Originally Posted by
pizzaguy
If you put an oil pressure gauge on the engine, you will never put 0W40 in there again. (Exception would maybe be a short-commuter in winter.)
5W40 is the MB recommendation since around 2011 and that's what I run. Back when I was running the Graphite really hard in hot, Georgia weather, I went with 5W50 - but it was pretty heavy below around 60F.
Every approved oil is on this list - the list changes over time, but right now, here is what MB says - notice the emphasis on 5W40:
https://bevo.mercedes-benz.com/bevolisten/229.3_en.html
I think that using the oil pressure gauge to decide what oil grade to use is not of any great use.
You have stated before that the pressure goes up as the revs increase, this is to be expected as the pump cannot push the same volume of oil through the small clearances and holes at the same rate as it can when the revs are lower. This is because the clearances and holes have not changed while the volume of oil that is trying to be pushed them has increased in proportion to the increase in revs, ignoring small inefficiencies, therefore the back pressure increases and the oil gauge gives a higher reading. Having a high reading could mean that insufficient or no oil is being pumped through the small clearances only through the holes because the oil is too thick.
What you want is oil able to get through the small clearances no matter the temperature of the oil and engine. Also the oil must be able to adhere to the surfaces under load and not get squeezed out.
A basic understanding of the SAE viscosity numbers is a good start.