SM has lower maximum limit than SG, but current common offerings of SG like havoline probably have very little ZDDP juudging from their low TBN in their product data sheet. Oils dont have a minimum limit so in reality a low quality SG can have lower ZDDP than SM, BTW the ZDDP or SAPS limit are only till w30, w40 and w50 oils even with SN dont have to comply with SAPS limit, this is possibly true for SP too.