Octane is a rating figure and indicates the gasoline’s resistance to pre-ignition. The higher the octane figure, the higher the resistance to uncontrolled pre-ignition, called “knocking”.
Ethanol has a high octane rating (about 113 RON). E10 gasoline, is standard gasoline blended with 10 % ethanol. It is a common believe this results in a higher octane rating for the gasoline.
But, the ethanol can absorb water and will separate from the gasoline. You can find the ethanol back at the bottom of the tank. In this case the octane figure can drop with 2,5. The gasoline indicated as E10 octane 98, then gets an octane rating of 95,5.
Instead of boosting the octane figure, blends with ethanol might give a worse result. The can end up as a sub-octane fuel once in your vehicle.
Which octane rating your gasoline needs to have, depends on your engine.
The higher the compression, the higher the combustion temperature. A higher temperature means a higher risk of uncontrolled ignition. The resulting detonation (knocking) will damage the engine. Definitely to avoid.
For the Royal Enfield:
As RE drivers, we are quite lucky. The engine of the Royal Enfield is a low compression engine. The Bullet 350 has a compression ratio of 7,25:1 and the Bullet 500 has 6,5:1 (2000 specs). The EFI Classic Bullet 500 gets a compression ratio of 8,5:1.
From the graph , we can see than for our Royal Enfields we need to have a gasoline with at last an octane number 87 (RON). When this one is in worst case an E10, the degradation brings the RON we need at the service station, to at least 90.
No worries for our Royal Enfield. We can use 95 octane gasoline, even in an E10 version.
RON= Research Octane Number