The only high mileage oils I know anything about are Mobil 1 and Valvoline MaxLife.
Compared to their regular 10W-30, Mobil 1 10W-30 HM is actually a heavy duty oil. It has a ZDDP of 1000 compared to 800 of their regular oil. It has a HTHS of 3.66 compared to their regular oil with a HTHS of only 3.09. The 100C cSt is 11.79 compared to the regular oil with a 100C cSt of only 10. There are extra antioxidants, seal conditioners, and detergents. Because of the higher HTHS it is not rated energy conserving. Because of the extra ZDDP, it is not rated SM.
Valvoline choose to keep their synthetic MaxLife(and their semi synthetic) SM. They kept the ZDDP at the 800 level and added a healthy dose of moly(.028) Their HTHS is up to at least 3.5(A3) and their 100C cSt is 11.7. They also have the extra seal conditioners, detergents, and antioxidants. They also "HINT" at some "secret" anti-wear agent. Speculation is that it is either some form of ester or some way of keep ZDDP from depleting as quickly.
Both Mobil 1 and Valvoline stress that the seal conditioners thy used are just that, conditioners, not swell agents. Both say you can use the HM oil in a new engine, but, Mobil 1 warns that you would have a warranty problem as they rate their oils as SL. Valvoline makes a statement on their web that you can use their MaxLife in a new engine with no warranty issues.
I am inclined to think that the HM name is just a way of marketing a Heavy Duty Engine Oil without having to use HDEO in advertisements or name. Who wants to give the impression their regular oils are not that great so use this? In no way do I consider 75,000 to be high mileage.