I just bought a 1981 F350 with the 400 (6.6L). I don't have an owner's manual so I'm wondering what is the recommended engine oil viscosity. Thanks for any help

 

I don't know remember the book exactly but it is dependent on your climate.
Colder climes go towards the lower end, say 5W-30 or 10W-30.
Down here in TX I like a 15W-40, which would serve you prob anywhere but Alaska.
It is hard to go wrong with a modern multi-vis. You could get in trouble running a staight 40 in North Dakota starting in a blizzard.

 

10W30 this winter and 15W40 in the summer, or 10W40 year round.

 

if the motor has a few miles on it like mine and is starting to get tired. u might use a little thicker to keep the oil pressure up say 15w 40 in the winter and 20w 50 in the summer