islandbeef

alx

One only needs to run octane number that prevents engine from pinging and detonation. The engine listens for pinging via microphones in the block. If premature ignition is detected the ignition timing is retarded. However, retard is not infinite. If maximum retard is obtained and there is still pinging serious piston damage can occur.

Retarding timing results in diminished power, engine response and increased fuel consumption.

So if you put 87 (93 in Europe) and there is no audable pinging when engine is loaded- and you can live with the power and response- you are good. High outdoor temps, excessive engine load (towing or prolonged driving uphill) will increase chance of detonation. So ymmv.

German engine controls have somewhat limited retarding capabilities and aggressive timing advance in part throttle situations. So higher octane fuel is recommended and some times truly required.

BlownV8

The higher performance MB's really do require premium while the NA GL 450 works perfectly fine under normal driving conditions with regular. As ALX stated, premium will be required under curtain circumstances and is probably recommend if you are towing.

a2j

Lower octane fuel is "easier" to ignite. Putting lower octane fuel than required by manufacturer will result in engine damage from premature ignition. It isn't about power and fuel consumption, its about design of the engine. High compression naturally aspirated engines and engines with forced induction require high octane fuel. Some vehicles can run on both, like flex fuel vehicles. If I'm not mistaken, Mercedes says it's ok to put regular gas in Metris vans. But they recommend premium for those turbocharged engines.
People who fill up with premium to get better mpg are wasting their money if their engine was designed to run on regular. Premium will make their car slower and increase fuel consumption.