Have an 04' F-150 5.4l 4x4 and was looking at switching viscosities. Mainly because I was talking to an 06' 5.4l owner and he changed to a 5/30 viscosity and found that his mileage got better by 1 mpg. Any merit or disadvantage to a viscosity switch? If any body has run into this please let me know.

 

not sure what you're running now? I thought Ford recommended 5/30 for that make and model already. I don't really like changing viscosities - the engineers know what is best for your engine, you can do your part by changing the oil on schedule and keeping it topped up. Just my .02

 

I would suggest keeping the weight oil in your truck that the engineers and designers that built the motor suggest to put in it.

 

I ran the recommended 5w20 for close to 100k miles in my '04 F-150 with the 5.4 and got great mileage. Lighter oil for close tolerance engines and the mileage should be better with a little lighter oil. Switching to a 5w/30 should not hurt but it's not what's recommended.