My SL is D-Jet. In order to do any kind of tuning, they need a way of analyzing exhaust for %CO. (AFR can be translated to %CO). That is why I installed it initially. It uses a wide band O2 sensor.

I set it more for performance than economy, but with the gauge it is easy to try different settings for both idle and load mixture (different controls)

Those engines sometimes have a hot start problem. The gauge shows that up clearly - mixture goes way lean after car has been sitting hot for a short while. Enriching idle mixture helps overcome that.

Sorry - probably more than you wanted to know

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5