dnewton covered the MM factor/aspect with this MC labeled filter with W suffix very well. It would be no different a non MC aftermarket OF as far filter warranty coverage goes. In short a dealer or manufacturer can not say a specific brand ie., oem "must" be used or they would have to pay for that specific filter.
Preface by saying, I have not seen the W suffix MC filters, will look next time I'm in WM. I'd be curious to see a comparison c&p of the W suffix vs dealer sold MC filter. My "speculation" is the W suffix is still identical to the MC sold at the dealer. With this development if across all applications, it seems as though dealers got tired of WM selling identical MC filters for less than the dealer and pressured MC (or powers that be) to add some kind marking to identify non dealer sold MCs. And reading an explanation above, perhaps as an excuse, lame as it is, to deny warranty service. Again, that just my speculation based on known information and what posted in this thread. My .02