One of the biggest differences is the lower level of ZDDP antiwear agent in SM motor oil to address autmaker concerns about premature catalytic converter falure from phosphorus "poisoning". (the issue of "premature" being whether the replacement converter is on the automaker's watch vs. the owner's) To compensate, SM adds an additional antiwear agent - typically molybdenum, though there are others to choose from. When SN is eventually released, levels of ZDDP will be reduced further. Base oils in conventional formulations were further enhanced from SL to SM. Many SL oils were a combination of Group I and Group II, with some upper tier conventionals being totally Group II. Virtually all SM conventional motor oils in 5W-20, 5W-30, and 10W-30 viscosities are at least Group II/Group II+ blends, with some being Group II/Group III blends whether they state "synthetic blend" on the labeling or not.