the issue is NOT fuel economy.
because the M103 has a high compression ratio, it requires a high octane fuel. If you use fuel with an octane rating lower than that recommended, and the engine is placed under a high load, your engine will "knock".
Knocking occurs when the fuel pre-ignites due to the effects of compression (like a diesel), and this is bad for the engine. Imagine this happening as the air-fuel mixture is being compressed, before the piston hits top-dead-center on the compression stroke, and you can imagine the stresses on that piston and those wristpins.
This can be heard as a "pinging" noise when the engine is placed under load. Hence the precautions about gentle use of the throttle if your fuel has an octane rating below that recommended.
i give my car what it wants, but i also sometimes drive it hard ...