You MUST check your owner's manual. It will specify the exact grade and specification of oil recommended for your particular Fiesta TDCi. The manual will often list several acceptable grades (e.g., 5W-30, 5W-40) depending on the ambient temperature.
Using the wrong oil can seriously damage your engine. Don't rely on general advice; always consult your owner's manual. The information printed inside is specific to your vehicle's engine and is crucial for its longevity.