Do you mean between black and light grey? Light grey pins have always been the kind you use for rotating connections (low friction), whereas black was for non-rotating ones (high friction). Newer blue pins are also high friction, IIRC. I haven't bought new lego technic in a while, so I don't know if there's been any new colours added

EDIT: I think I also had some dark grey pins, but I don't remember if they were high or low friction

> Light grey pins have always been...

I think the black ones were a later addition, likely late nineties.

My memory of twenty years ago says the dark-grey pins were 1 stud wide on one side, and half-wide on the other, and low-friction like the light-grey ones.