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

The recommended oil weight for a 1966 Ford F100 with the 300 cubic inch straight-six engine is 20W-20. However, depending on climate and operating conditions, 10W-30 or even 20W-40 might be acceptable. Always check your owner's manual for the most accurate and up-to-date recommendation for your specific vehicle and location. Using a multi-viscosity oil like these is generally preferred to a single viscosity oil (like straight 20W) in modern applications.