M1 is what I tried using in a 24 HP INTEK purchased new B&S engine on a Craftsman DYT 4000. At 8 hours of use the left exhaust valve locked down from all the gook on the valve stem/guide. The inside of the head was coated thick with a goo coke substance this is oil that went past the rings, when burned in the combustion chamber caused a coke deposit that coated the entire head/chamber area, the top of the piston, the galleys inside the head where oil flows to the rockers. Also there was a black gooey type mess in parts of the bottom end, it was between a sludge, and coke more like tar.. All of this could be said to be caused by poor engine construction, defect etc, however I have seen the exact same thing in another B&S engine which was catastrophic at around 80 hours... this engine had wore the area of the crank/connecting rod/journal which I don't attribute to the M HOWEVER the exact same tar like goo was also in the bottom end of this engine, and also the valve stem, the head/chamber area, and also piston top was coated... this is not the normal carbon coating that effects all air cooled engines..
This is my experience on this subject, and I assume this is also why B&S elected so long to use 30 wt oil, I assume people having to HAVE a synthetic oil to use may be the reason why B&S makes a synthetic oil now from my reading is just a older type oil formulation mainly in the additive section... Which probably still is not as good as the HDEO 30 wt oils I posted above..
It does not bother me whatever anyone decides to use, however I would like to save people the trouble of what I have had...