Modern engines have knock sensor(s) and at least two spark advance maps. Since MB engines have fairly high compression ratios, premium fuel is required to run the most aggressive spark timing map for best knock-free performance.
If the knock sensor detects persistent detonation, it will default to the less agressive advance map and this is what will happen if you run regular unleaded (87 PON) fuel. The knock sensor will detect light detonation that you probably can't hear.
Fifteen years ago most OEM owners' manuals had dire warnings of engine damage if lower than recommended octane fuel was used. Now they usually just recommend, say, 91, and say that if you use a lower octane you may notice a loss of performance. This is because of the less aggressive timing when lower octane fuel is used, but you won't harm the engine.
I've recommended a number of owners of several makes who were using the recommended premium fuel to try 87 octane fuel and none of them even noticed a performance difference. I got tired of hearing them whine about having to buy premium fuel when gasoline prices were high.
To satisfy yourself that using lower octane fuel will not cause detonation try the following experiment. Let the fuel get close to empty and add five gallons of regular unleaded. Drive the car and listen for detonation and determine if it has a noticeable loss of performance. If you detect significant detonation (which you probably won't) fill it up with premium. If not, when it gets close to empty again add five more gallons of regular unleaded and continue the test. By the time you consume the second five gallons of regular unleaded you should know if the engine knocks or you experience an unacceptable loss of performance. If you find no adverse affects, fill it up with regular and continue to use it.
The only difference between regular and premium fuel is octane rating. All modern gasolines have detergent additives to keep your fuel system, injectors, and valves clean.