However, generally speaking, for a 7.3L Power Stroke:
* 15W-40: This was a very common and often recommended viscosity for this engine, particularly in normal operating temperatures. Look for a CF-4, CG-4, or CH-4 rated oil (or better). These are older classifications, but still relevant for this older engine. Modern oils like CJ-4 and CI-4+ are backwards compatible.
* Consider the operating conditions: If you live in extremely hot or cold climates, you might need a different viscosity. For extremely cold weather, a 5W-40 or even a 0W-40 might be necessary. For very hot weather, you might consider staying with 15W-40.
Crucially: Always check your owner's manual for the manufacturer's specific recommendations. The manual will specify the viscosity grade and the API service classification that's best for your particular truck and engine. Using the wrong oil can seriously damage the engine.
If you don't have the owner's manual, you can often find a PDF version online by searching for "1999 Ford F350 owner's manual" along with your engine size.
