As edyvw pointed out to me in a recent thread, 229.51 & .52 are low saps oil which are required if the car has a GPF.
Sufated ash is not an antiwear compound but he advised that I NOT use a low SAPS oil in an MB calling for 229.5 or 229.6.
Here is his post, hopefully it will help:
"Lubrizol also tells you not to compare specifications with each other.
MB229.6? Specifies both? Which ones? MB229.6 and?
In reality, MB 229.51/52 could be used in the US after 01/2017, when we also switched to ULSG. In Europe, MB has been using 229.51 in gas engines since 2009 (in some countries, mostly EU (some not) and Switzerland, Iceland and Norway). But some MB are strictly MB229.5 and cannot use MB229.51. The difference is that with ULSG TBN won't be depleted before 10k OCI. That does not mean MB229.51 "replaces" MB229.5. Both have their place in MB lineup and both are introduced at the same time and both are updated throughout the years with new specifications at the same time.
MB229.6 is ACEA A5 oil. It is Full SAPS (there is MB229.61, which is Low-SAPS oil) and lower HTHS oil than MB229.5. It is basically "energy conserving" oil.
I think at one point, MB had an explanation in the manual on when to use MB229.5 (maximum wear protection), when to use MB229.6. In some, it had an explanation of when to use MB229.51 (maximum wear protection), when MB 229.61 and when MB229.71."