High mileage oil contains additional additives and more detergents than regular oil that are designed to clean out sludge inside the engine and the additives are meant to reduce wear on moving parts.
As an engine gets mileage put on it parts wear out and need those additional additives and detergents to help protect the engine and the parts from wearing out.
If your vehicles engine has more than 70,000 miles on it then it would be a good idea to use high mileage oil in the engine to help make the engine last longer.
The high mileage oil also helps protect the seals as well and helps to seal off any bad engine seals to help keep the oil from leaking out when it shouldn't be.
The high mileage oil costs a bit more than regular motor oil however the cost of the high mileage oil will save you even more money in the long run because your engine will last longer.
An engine or another vehicle costs much more than the cost of the high mileage oil would cost.