Entirely depends on the fuel station as there are good and bad examples of each brand of fuel... BUT
BP Ultimate both guarantee's octane (98 octane as the minimum octane rating of the batch, as opposed to other brands listing '98 octane' as the average octane of the batch), and run the least amount of cleaning agents in the fuel.
V-power is the only other fuel I'll personally use, which I find gives me slightly better fuel economy but less mid-range. BUT, they also have the highest proportion of cleaning agents... so I often use V-power as a once every so often fuel to clean out the injectors etc.
I've had an occasional batch of ok Caltax 98, but generally it's been pretty poor... and Mobil 98 was always horrid.
These results were consistent across both my 98 octane tuned turbo cars (Polo and WRX), however I find the results harder to differentiate between the NA cars I've driven which require only 91 or 95 octane.
Previous Rides: Polo GTI, Mx5 10AE, MY05 WRX WRP10, Renault Sport Clio 172
Current Ride: Evo 8 MR, Fabia MK3
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