Consult your owner's manual for the most definitive answer, as Ford's recommendations might specify a different weight depending on conditions. If you don't have the manual, you can often find a digital version online by searching for "1999 Ford E-250 owner's manual."
While 5W-30 is a good starting point, using a 10W-30 in hotter climates is acceptable, but avoid using anything thicker than that unless specifically recommended by Ford for your vehicle's operational conditions. Using too thick of an oil can lead to increased engine wear. Conversely, using an oil that is too thin can lead to insufficient lubrication, especially in hot conditions.
