The correct engine oil for your 2010 Ford Focus TDCi depends on the specific engine code and the Ford specification. You
must check your owner's manual for the exact recommendation. The manual will specify the correct viscosity grade (e.g., 5W-30, 5W-40) and the Ford specification (e.g., WSS-M2C913-A, WSS-M2C913-B, WSS-M2C934-A).
Using the wrong oil can severely damage your engine. Don't rely on general advice; always consult your owner's manual. The engine code is usually found on a sticker in the engine bay or in your vehicle documentation.
