Mobil's 5W40 synthetic oil is one of the most recommended engine oil for the Mercedez-Benz CLK430 due to the considerable downtime required in servicing the flexible service system. This engine oil is compatible with the 1999 through 2003 models.
The flexible service system monitors the engine oil life of the vehicle and provides notice to the driver when that system needs maintenance, according to MBWorld.com. Mobil 5W40 synthetic oil improves the performance of this system and helps ensure that the driver only needs to change the engine oil every 20,000 miles. While 5W30 synthetic oil is also an acceptable choice for this vehicle, drivers should steer clear of any multi-blend or non-synthetic formula engine oils.