Wow. That is interesting. The voa I did (link above) is on the $13 version and that is the version my dealer uses. Look at the back of the bottles in the ECS photos. The $13 version is an API SM rated oil and the only "approval", if that's the word, it carries is for M models only. The less expensive version is an API SN rated oil and also includes a recommendation applicable to VW/Audi engines. SN oils are the so-called low SAPS (sulphated ash and phosphorous) oils, many of which are formulated for use in countries with low sulphur gas (e.g. many European countries) and therefore tend to be low in TBN (reserve alkalinity) and somewhat low in zddp - - IF (and this is a big if) the oil is the more prevalent 5w30 or 0w30 weight. At 10w60, I would think BMW/Castrol could do whatever they want irrespective of whether they call it SM or SN.
It may be that the SN oil is for use in Europe or any country/State that has low sulphur gas. But I'd for sure do a voa on the SN oil before using it. Maybe also talk to ECS to get their take on the difference between the two. In a performance oil, SM is the older standard and generally gives the formulator more freedom in mixing the add pack, particularly in the more prevalent 0w30 or 5w30. The BMW High Perf. oil used in the newer M turbo and non-turbo cars is a 5w30 but rated under the old API SM std just to give Castrol more freedom in the formulation (the APi stds applicable to 0w30/5w30 oil in SN are much more restrictive).
Hope some of this makes sense. The SN std does have its plus side. The SN oils are generally much cleaner, less additized oils, that burn cleaner and leave much less deposits in the engine. So in some circumstances the SN TWS may be fine.
09 Z4 '02 M3 '91 328i '90 535i
'05 M3 ZCP '88 528e '88 M5 '04 Mini Cooper S
'87 M6 '95 Z3 '86 635i
'05 330i ZHP '93 M5 '87 325e
'89 535i '95 M3 '74 Bavaria