can you mix 98 and 91 petrol

Yes, you can mix 98 and 91 petrol; your tank doesn’t explode, it just ends up with an average octane—so performance and knock resistance drop accordingly.

If your car only requires 91, fill your boots; blending with 98 is harmless, just mildly wasteful. If the manufacturer specifies 95 or 98 minimum, avoid 91; in a pinch a small top-up of 91 is usually okay, but drive gently and return to 98 ASAP.

Modern ECUs can pull timing to protect the engine, but that means less power and possibly worse economy—so no heroics. Watch for E10 labels: many 91 pumps are E10, which is fine only if your car is rated for ethanol.

Never mix petrol with diesel (obviously), and if the engine knocks or runs poorly after a misfuel, don’t keep driving—refuel correctly or get it drained.