Multigrade detergent oils got a bad name when they first appeared because, frankly, they weren't very good. A spell of fast motoring in hot weather and the oil film broke down - resulting in a seized engine. The oil companies had also overlooked the fact that their new detergent additives would scour accumulated carbon deposits from dirty, high mileage engines - causing disastrous blockages and yet more engine seizures. By the end of the fifties, though, when the Mini appeared, multigrades had improved vastly. Technology doesn't stand still, of course.
Car engines keep changing, and the oil companies have to develop new products to keep pace.
The trend nowadays is towards fuel-efficient, low emission engines. Built to tight tolerances, they have small sump capacities so the oil gets up to temperature very quickly. Today's premium synthetic oils are designed for today's cars. They're technically brilliant - but, unfortunately, they're too thin and too highly detergent for older engines. For most people, there's an easier option.