Premium or nothing, PERIOD. Especially in older S40's.
End of story.

 

There is little difference in energy content of regular versus premium gasoline. They both contain about 111,400 British Thermal Units of energy per gallon. The experts consensus goes against the long-held belief by thousand of drivers who fill up with premium only or on every third or fourth trip to the pump.The idea is to fill up with premium every so often to clean out the engines or rev up the performance of older engines.