Esso Super Unleaded

[Mr Morse]

Optimax is 98 RON.

Best to use another brand every 3-4th fill as Optimax has been known to leave a film behind.

[flotsam]

Normal unleaded isn't "watered down".

95 RON self-ignites at a lower compression than 97 RON which is what Japan uses. This is how diesel engines work; by self-igniting; no spark plug.

If your engine has a compression ratio of about 10:1 or higher, you need the higher RON "super" fuel. Otherwise, the engine will compensate with its anti-knock/pinking sensor and back off the timing. The whole idea of a higher compression is greater fuel efficiency. But if the timing is backed off it loses this advantage.

Fifth Gear showed a Subaru Imprezza makes full use of super unleaded and gained several percent in power and torque.