Owners repeatedly describe the M120 V12 as one of the best production twelves ever, extremely under-stressed, and good for very high mileage when maintained. Multiple owners report 100k-200k-mile examples still strong; the internals are considered essentially bulletproof, with problems coming from the surrounding luxury systems rather than the engine itself.
The M120 6.0 V12: robust, under-stressed, and long-lived
Mercedes-Benz S-Class Coupe / CL (C140) · mechanical