Just to confirm - this is the official line from both BP and Shell regarding their premium Unleaded petrols:
BP:
Dear Mr Moffat
Thank you for your query received in this office regarding BP Ultimate fuel.
The RON (Research Octane Number) for BP Ultimate Unleaded is guaranteed 97. For Performance vehicles it is also helpful to know the MON (Motor Octane Number). This is where the Antiknock quality is tested at full throttle and High engine speed, and part throttle and low&high engine speeds. The MON for Ultimate Unleaded is guaranteed 86, however it is typically higher, upto 87.5.
BP Ultimate burns more efficiently and cleans your engine as you drive to deliver sharper acceleration and improved responsiveness while producing less harmful emissions. Compared with ordinary unleaded, the benefits of BP Ultimate come from the properties of both the base fuel and a number of other special components added to the formulation. However, don’t forget that BP Ultimate is the name for two new fuels - one diesel and one unleaded and the formulations for each are different.
Thank you for your interest in our new advanced fuels.
Yours Sincerely
BP Customer Care
And Shell:
Thank you for your recent e-mail.
We guarantee that the RON of Shell Optimax will be at least 98, typically it is refined as high as 98.6.
The MON is guaranteed to be a minimum of 86, but more typically 87.5.
I do hope this answers your question.
Kind regards,
Kathryn Atkinson
Fuel Quality Co-ordinator
Shell U.K. Oil Products Limited
Rowlandsway House, Rowlandsway, Manchester M22 5SB, United Kingdom
So... Optimax IMO, is the better fuel. Cheaper, a higher RON and probably the same level of MON.
I for one noticed better performance and better fuel consumption when I changed from normal unleaded to Optimax in the S3.