[Conehead]

To answer why some owners suffer from this, you have to find out what the spec of the car is really. Perhaps it's a sensor fault, or perhaps the map they use has been taillored to what they expect?

If it has been changed from the manurfacturers spec and not been looked after then you will always have a risk of stressing engine components.
Most of the cars that appear to suffer with this have still got distributors, and would not suffer if they were full sequential injection systems.

You also need to remember that some of these cars are 15 years old with 150000 on the clock, which volvo or not is a lot of miles, and things start leaking, corroding and generally break down.

I'm not saying that you should use one type of fuel or the other, I am saying however that a standard spec car should run on normal unleaded without any of the problems, in the country that the car was designed to be used in.
But just don't expect to get 240 bhp on asda unleaded.

You also have to consider , especially after recent events, whether the fuel is quality controlled enough to worry how it would affect the car's engine.

FWIW I agree if you are going to chip your car then you like the power so why use anything but super or v-power or Ultimate? You don't have to but why would you pay for the chip and not get all it has to offer?

I'm not trying to butt in and cause/cure any arguments, I just felt that maybe for the less knowledgable I could maybe explain why different fuels effect the engine in different ways.