High Mileage oils use to be a thick for grade oil, with extra seal conditioners (which are in regular oil anyway - but to a lesser amount) and often with elevated ZDDP levels (API SL) for extra wear protection.
But now with many insisting on only using API SN oils, the ZDDP zinc levels have been reduced to regular ILSAC amount, and the seal conditioners don't work on many modern seal materials. So what you are left with is a slightly thicker oil, with a big font HM on the label.
Strangely enough HM oils have never been popular in Australia, we simply step it up a viscosity grade as the car ages.