From my understanding, the key thing to focus on with HM oils are the seal conditioners and detergency. Awhile back, an XOM sales guy posted a few things about HM oils. They are basically boosted in the seal conditioner (which are not just esters and vary from oil to oil) and detergent area. Unforutunately they don't reveal much about the seal conditioner part. Why should they? I think in most cases they are a form of diester, but I'm not sure that is what Ashland uses. These oils also have higher ZDP/Ca/Mg. So despite the original creation of a high milage oil being sort of gimmick, these oils do have a purpose and do work for specific reasons.
*Defy does have added seal conditioner which makes it a HM oil.
Thanks Tom!