HM oils have a larger dose of seal conditioners. Other things that differentiate it is a tad more ZDDP, most may be on the thicker side of the specified weight, more detergent is definitely included and makes the oil act like a mild flush, and you may even see a good dose of moly. I think many HM oils were API SL up until recent years (correct me if I'm wrong). Mobil 1 HM recently became an API SN oil. The only HM oil I can think of that is API SL still is Royal Purple HMX.