[throttlemeister]

Higher octane fuels are better resistant to detonation caused by high compression. Unfortunately, it also burns slower than lower octane gas, so you need more ignition advance to get the same power output (basically getting the 'bang' at the right time). Now there lies the problem: if an engine is designed and setup for high octane fuel, putting in low octane fuel will make it more susceptible for detonation due to the high compression, and the increased ignition advance will make it even more prone to detonation.

Fortunately, all newer BMW's like the R12S have a knock sensor, which will retard ignition if it detects detonation. If the octane rating is low so that the knock sensor adjusts timing due to fuel octane, but high enough to prevent detonation due to high compression, like 91 should be in most cases, there will be no performance loss. Performance loss will only occur if the advance has to be retarded beyond the optimal for the fuel burn to avoid detonation. Mind you, there should also not be any performance gain.