[joebris]

it appears that bp and shell both have cleaning agents in thier 91 fuel.  Caltex not so,  will keep looking

[Dazzler]

If your car doesn't knock or ping with 91 I would just be using that. If you like the idea of the extra cleaning additives in 98 then maybe use that every 10 fills or so! Here in Tassie (last time I checked) 98 was 10c or more per litre dearer than 91 octane. I'm not convinced you get extra distance out of it.