Important Considerations:
* Check your owner's manual: This is the most authoritative source for the manufacturer's recommended oil type and viscosity. The information above is a general guideline, but your owner's manual might specify something slightly different.
* Climate: If you live in an extremely hot or cold climate, you might consider adjusting the viscosity slightly. For extremely hot climates, a 20W-40 might be considered, and for extremely cold climates, a 5W-30 might be better. Again, consult your manual for guidance on this.
* Oil Type: Stick with a conventional oil. While synthetic oils offer some benefits, they weren't as widely available or necessarily as beneficial for older engines like your 1991 F-150. Using a high-quality conventional oil is perfectly suitable.
* API Certification: Make sure the oil meets the API (American Petroleum Institute) certification requirements for your engine. This information will be on the oil bottle.
In short, 10W-30 conventional oil is a safe and effective bet, but always defer to your owner's manual for the most accurate information.
