Yes, Mercedes recommended premium fuel, which is 91 PON here in California.

Back when I drove the car daily (and it still had the OE 750K R16/1) it would sometimes detonate on hot days when the coolant temp was over 90 and the A/C was on, so their recommendation was probably correct.

Now I only drive it a few times a month during the winter and spring months when ambient temp is usually no more than 70, and I rarely use the A/C, so 87 is okay.

Detonation is really senstive to inlet air and coolant temperature, but a little transient detonation, which is what I get sometimes, means the spark advance is near ideal.

If both the air temperature and coolant temperature are high I can get around any detonation by shifting at higher revs and easing into the throttle. It's strictly a low rev transient phenomenom. The EZL may be a little slow to back off the vacuum advance on upshifts.

If you're not experiencing any detonation on 89 try 87, and if it doesn't exhibit significant detonation on 87 you can continue to use it. If my car had an auto I doubt if I could detect any detonation because it only happens at below about 1500 at high load, which is not an operating condition a torque converter will allow.

It's really difficult to determine the ideal spark advance map unless you can change it easily and use a high octane fuel on a chassis dyno to determine the optimum advance for all operating conditions. Often it's near the detonation point if you have marginal octane for the specific CR and combustion chamber design, but given the improvement in both low end torque and fuel economy with the R16/1 removed plus the occasional transient detonation, I think the advance is pretty close to ideal in a design that doesn't have a detonation sensor that will tend to keep the advance right on the ragged edge of detonation for all operating conditions. That's how most modern engines are set up.

Duke