My mate at work has only had his GSI for a few weeks.
He get's through petrol fairly quickly, and has filled up the car 6-7 times in the past four weeks.
He has tried running it on Optimax for 4 tankfull's and is now running it on BP Ultimate.
He prefer's the BP Ultimate, as he says the car is smoother and pulls better.
I myself have used Optimax for as long as I can remember, and filled up with Jet Super last night.
I tried giving it some stick, and the car did seem to rev better.
Nikon run's his car on Super Plus only and got a good power figure at Northampton, is optimax overated...?
I must say with optimax, after it has been in the car nearly a week the performance does go a bit flat.