Changed from using Shell Optimax to Mobil 8000 and my figures have changed from 8.9l/100k to 8.3l/100k (average of last 5 fill ups) for a stock 2006 manual standard with just over 12,000km on the clock.

Driving habits not significantly changed (but weather is a bit warmer) so I am happy with the change which is about break-even. It does seem to be "more responsive" and have a bit "more power" measured by the famously accurate "butt meter" and how much throttle is required to maintain a given speed.

May give BP Ultimate a go and see what that does.