Your friend's brother is right--it's meeting the 229.5 specification that matters. Oils meeting 229.3 are also fine (and approved) for use in the M112/M113 engines, although oils meeting the 229.5 specification (which wasn't even established until years after the M112/113 engines were designed and introduced) are slightly better and thus preferable. MB's official list of oils approved as meeting 229.5 includes many 0w30, 5w30, 0w40, and 5w40 oils. If in doubt, look up https://bevo.mercedes-benz.com/index.php?language_id=1
General recommendations about which oil viscosity to choose depending on the range of temperatures in which the car will be operated can be found in the car's manual (although they, naturally, date back to the moment of production). Note, however, that oils in one group ("5w30," for instance) still vary greatly in their performance, which, evidently, includes their viscosity at different temperatures as well as their viscosity indices. That's one reason why sticking with the MB specifications is important.
The use of (the correct) paper filters in M112/M113 running the FSS is fine (and approved), too. Synthetic filters are required, however, if you want to run these engines on extended oil change intervals, i.e. 15,000 miles instead of 10,000 miles. Nonetheless, many drivers (including me) use synthetic filters even when running the normal, 10k miles oil change interval as interpreted by the FSS. Synthetic oil filters are also required for the newer, post-2005 MB gasoline engines.