Look for an oil with a rating like 5W-40 or 0W-40. The first number (0 or 5) indicates the oil's cold-weather performance – lower is better for cold starts and easier flow in frigid temperatures. The second number (40) indicates its performance at higher temperatures.
Don't rely solely on the number: Always check your owner's manual for the specific oil type and viscosity recommended by Ford for your 2005 F-250 diesel engine. Using the wrong oil can lead to engine damage. The manual will likely specify an API (American Petroleum Institute) certification and potentially a CJ-4 or CI-4 rating.
Using the proper oil is critical for reliable cold-weather starting and overall engine protection in winter conditions. If you're unsure, consult a qualified mechanic or your local Ford dealership.