Just trust me, it's 5W40.
It's never been 30 for the M156, that would be too thin when up to temperature.
You can use 0W40 but 5W40 is preferable.
Any oil that meets 229.5 MB specification. This obviously includes their own but I run Shell Helix Ultra 5W40 which is regularly available cheap through places like EuroCarParts.
This was from way back in 2012 so the preference for 5 over 0 is long standing and came from Mercedes.
"Guys, AMG Private lounge answered this question: 5W40.
Lifters are prone to bleed down and 0w40 is too light at start up, making the condition worse.
Go hard if you want to use 0W40, but the suggested oil from the horses mouth is 5W40 and has been for years now."