For those of you who dont run Hi-test
For those of you who dont run high-test, you could seriously damage your engine through long term preignition(also called detonation). This means that the fuel ignites completely before the cylinder has reached full compression. It can and does cause engine damage, deposits, and other harmful effects. Consider this, The MB owners manual says that 87 octane is only for extreme emergencies, and that when filled with 87 octane, the throttle must not be more than 2/3 open. MB does not have any residual income from mandating usage of High octane fuels, so you should probably trust them in this case.