Q: weight of motor oil on 1999 Ford F-250 Super Duty

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what weight oil is to be used in a 1999 F-250 Ford 4 wheel V-8 pick up truck
(2) Answers
7.3L Diesel = 15W40 like hemicuda stated.

5.4L gasoline engine takes 5W20 (started in 2002, backwards compatible...)
Usually Diesel's run 15W-40 also I found this under your vehicles common problems reports on Repairpal

Use correct oil to prevent engine performance problems
Engine -- RepairPal Verified
Using the correct oil for this engine is critical. Improper engine oil can foam in the high pressure system and not develop enough pressure for operation. Symptoms of this can be hard starting, long cranking time before start, rough engine idle, and a lack of power. Another sign is tiny bubbles in the oil on the engine oil dipstick after the engine is shut off. The additives in the correct oil that protect against foaming can still break down, so changing the oil at the correct intervals is important.