I think it just means Full Synthetic, but I guess that tends to imply that only the 0W40 and 5W50 oils in the M1 line are truly full synthetic?
I think I have read here that only Group IV and V oils can be marketed as full synthetic in Germany, perhaps this is an indication that those oils meet that standard...my supposition and $4.99 will get you a small coffee at Starbucks, though.