I checked my manual and it states 10w-40 SF/SG is the oil that should be used in my 400K3. I went to Advanced Auto and looked at the automotive oils and everything was rated SM/SN. When I asked at the counter, the guy said he didn't know the differences between the grades but he had motorcycle oil. I checked it and sure enough, it was SF/SG grade so I got that (but haven't used it yet). My questions are, what are the differences, could I use SM/SN grade if they were out of motorcycle oil, and what should I look for if I wanted to use synthetic oil? Thanks for any help y'all can provide.