What weight oil is used in a 1966 Ford F100 300 cubic straight six engine?

Answer 1

For older engines, the recommended oil weight (or more properly put, viscosity rating) has more to do with climate and driving conditions than it has to do with make and model.

Entire treatises have been written on the subject, but to make it simple, most older engines do just fine on nearly anything including 10w30, straight 30, 10w40, or 15w40. Avoid the "lighter" stuff (such as 5w20 or 5w30) unless you live in the Arctic. Avoid the "heavy" stuff (such as 20w50) unless you regularly work the truck hard under hot conditions.

For a heavy duty motor such as Ford's 300 inline 6, your best bet will be 15w40 for most use, or 10w40 in the wintertime.