I can attest to this...
My car had it's PowerFC tuned for Optimax a month or so ago... and it was running beautifully...
The last two tankfulls of Optimax I put in, made the car detonate in 4th and 5th gears are high loads... I tried to remedy this by octane booster, but it still detonated...
Last night I went for a run of the OPH, and filled up a tank of Mobil Synergy 8000 and the car again, ran beautifully... No sign of detonation...
Shell must have revised their chemical composition of Optimax in their latest batches of fuel
Merli. :king: