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.
The differences as published by the API are as follows:
SM is limited to .06 min/.08 max percent phosphorous.
SL is limited to .10% max phosphorous.
TEOST is limited to 35mg max for SM
TEOST is not rated of SL.
Sulfur is .5% max (0w/5w) for SM
Sulfur is .7% max (10w) for SM
Sulfur is not limited for SL.
Engine test for SM is sequence IIIG
Engine test for SL is sequence IIIF
SM limits viscosity increase to 150% max at 100 hours.
SL limits viscosity increase to 275% max at 80 hours.
Cam wear using sequence 4A limited to 90 micron max for SM.
Cam wear using sequence 4A limited to 120 micron max for SL.
Bearing weight loss using sequence 8 is 26 mg max for SM.
Bearing weight loss using sequence 8 is 26.4 mg max for SL.
There are numerous other differences but they are measured using different test sequences and therefore not directly comparable.
There is no published limit on the level of zinc additive that I know of.