Got a lexus is250 in the family, always maintained by lexus. It was running a little rough (still smoother than most other cars mind!) Took it to lexus and were told to run it exclusively on premium unleaded. Problem cured!

So what were you running on before.?????..and what is their interpretation of "premium".?

They said don’t use the regular petrol. It was being run on the regular fuel, mainly from shell garages. By ‘premium’ I think we all know this means the more expensive of the two, unless you’re making a broader point?

What do they mean by "regular" fuel.?...95 RON IS premium fuel. (only termed as regular because it's the one we all mainly use)..unless you have a high performance car and/or one that demands a higher octane, then you are just chucking money away IMO.

If you want clean your injectors, run the tank down to 1/4 full and stick a bottle of injector cleaner in the tank. Run that until the reserve light comes on and then fill with ordinary 95 unleaded.

I have tried running one of my motorcycles on 98 super, absolutely no difference in performance or economy, so now everything petrol engined in the fleet gets 95 UL.

I think that it is the extra detergents that Shell claim to put in their top fuel that is more important than the octane rating

And there is the problem, its just a claim. Occationally they produce nice colour photo's of dismantled engines showing how much cleaner one is than the other but there is no proof that the engines involved had actually been used for 100,000 miles on 2 different fuels.

The most miles I have ever done in a car was 113,000 in a Golf. Had it form new, always used supermarket 95 RON and did quite a bit of towing during the summer months (race car on trailer). At 7 years old the emmisions were still factory spec at the MOT and when a few days later I was pulled for a DVLA roadside check they were stunned that it was so low on a car with that mileage.

So no 98 RON for me, why would I spend more when 95 RON does the job perfectly well.

In answer to the OP.....

........"Shouldn't 'premium fuel' be at least 98? Are we being diddled? " Yes and yes.

Thank you KJP for probably the best answer yet on this thread. I really should have searched on the forum before I posted as I have just discovered this was discussed nearly 13yrs ago!

www.honestjohn.co.uk/forum/post/index.htm?t=32849

Still, no harm done to revisit it.