Theoretically the typically higher concentration of P in CK-4 oils will "age" catalysts faster than the lower concentration in S-category oils, but "your mileage may vary".

I don't see a huge liability for you here, and it may be worth the convenience and cost savings of using bulk C-category oil. I recommend C-category for all cars with sliding surface valve train components like my '88 190E 2.6... don't know if your M112 is "roller everything", but that's what current S-category oils with lower concentrations of anti-wear additives are meant for.

Some CK-4 oils have much less than the 1200 PPM allowable P concentration because ZDDP has been reduced due to the advent of newer Boron based anti-wear additives. I don't know if Boron is an issue with catalysts, and maybe the jury is still out on the subject.

If you can find a spec sheet for the brand C-category oil check it out. I recall that Delo has reduced P, but they don't list a Boron concentration. The important thing to know is that all C-category oils have to pass the tougher than S-category wear test, and this can be done more than one way. Up to 1200 PPM P from ZDDP or less ZDDP supplemented with a new Boron-based additive both get the job done.

Assuming your E320 is performing well in emission testing and you're not driving it into the ground it may be exempt from emission testing due to age before the catalyst goes tango uniform or a major repair like a transmission overhaul may consign it to car heaven before the catalyst goes south.

I don't think you're taking a big risk using C-category; 15-40 is okay for consistent cold starts down to 15-20F, and I doubt if a difference in fuel consumption between 15W-40 and 0W-40 could be detected without laboratory equipment.

There's more information in my oil article. I wrote it primarily for the vintage Corvette community, but there's a lot of good FACTUAL information in it that blows up a lot of internet myths (like "they took all the zink out of oil").

https://stlouisncrs.org/news_files/St_Louis_NCRS_Chapter_July_2011.pdf

Duke