Hello,

Basically if your 240 uses fuel injection it tries to reduce knock (due to improper octane gas) via the knock sensor on the engine block. If your 240 is the carburettor version then it pays to use the proper octane gas because the engine can't adjust itself if knock occurs.

Octane rating is basically for compression engine. Non-compression engines like those in jet engines (mind you old fighter propeller aircrafts used compression engines last time) use no octane ratings.

In compression engine you want the air-fuel mixture to ignite when it is supposed to ignite (when spark occurs) and not earlier. Compressing air and fuel mixture automatically raises its temp and can ignite on its own (without the need for spark). Factor in the hot engine operation temp, the mixture could ignite earlier than intended thus producing a sound described as knock.

Anti-knock agent such as MBTE (or last time tetraethyl lead) is mixed with the gas to reduce this self-ignite. Nowadays MBTE is raising concerns in some country as it can pollute the underground water in case of leaking old underground storage tanks in gas stations.

You can mix those gasoline as what you did as long as the engine can manage (no knock). Conversely don't get higher octane than what is required for your engine as you're actually paying for more anti-knock.

Regards,
Amarin.