The manual will specify the recommended viscosity (like 5W-30 or 10W-30) and the amount of oil your engine requires. Failing to use the correct oil can damage your engine.
If you don't have the owner's manual, you can often find a PDF version online by searching "[Year] Ford E-350 owner's manual" on Google. You can also check the engine sticker (often located under the hood) for engine type information, which can help you find the correct oil specifications online.