I have a 2002 Mercedes Benz CLK320 with a V6 engine, and I would like to know if it's OK to use 87 octane gas to save some money. My owner's manual says that the minimum fuel octane I have to use is supposed to be 91, but considering today's gas prices I'd like to use the cheap gas if I can get away with it without damaging the motor. It makes sense to me if the manufacturer tuned the engine in the factory to get the best performance on 91 octane. But to my understanding, modern car engines come with knock sensors that automatically detect if there's any engine knocking due to low octane fuel, and will automatically retard the timing to make it run smoothly so how can the cheap gas possibly cause any damage? I'm pretty sure there's no difference in actual quality or purity between different octane fuels, so it's not like it would gum up my engine. I have a feeling that engineers are too conservative on paper and that it would probably be OK, although I might just lose some power.

Using low octane can cause knock specially if you have a high compression chamber. in the long run it will damage your engine and cost you more. second lower octane have less energy and you end up stepping on the gas deeper to reach a certain speed compare to a high octane high energy gas. Its just this simple if you can buy MB 2002 CLK320 you should be able to buy 94 octane gas for it.