Unleaded or Super Unleaded in a 1999 528 Auto?

[sparks_E39]

I've ran my car on Tesco Unleaded for the past 6 months, I thought I might ask here if it makes any difference top these cars, and if so what, by using super unleaded? Is it worth the extra cost, what sort of MPG gain, if any can I expect and will the car perform better? At the moment I am doing a lot of short journeys, around town mostly less than 5 miles and the car has averaged 22.9 mpg over 1500 miles in the last 8-10 weeks.