Freedom is not free
Join Date: Sep 2007
Location: Sioux Falls, South Dakota
Re: 5-40 vs 15-40
As you 6.0L owners know - the 6.0L runs some pretty high ICP
and can shear just about any engine oil before 5K. Here is my 2 cents.
If you are operating in a cold temp environment then you will benefit from the cold start performance of a 5W-40, otherwise I would lean towards the 15W-40. Both 5W-40 and 15W-40 work well in the 6.0L, but on average the 5W-40 oils tend to shear quicker than the 15W-40of the same brand in this application. Basically the 5W-40 oils have more polymers to make it a 40 weight oil at temperature than a 15W-40. Under high pressure & temp the polymers can start to break down.
You can try the 5W-40 version and do an oil analysis at 4K and see what the results are yourself, but here is one that was just posted recently on another forum. The 5W-40 Valvoline showed very good wear results but it broke down from a 40 weight to a 30 weight by 3,680 miles (CST viscosity at 100 deg C
) dropped below 12.5. I'll see if a can find one of a 6.0L running the 15W-40 Valvoline.
2003 F250, Wildman tunes, Faubion trns, Swamps 175/146 SS,van turbo/WW, DI Intercooler, oilguard bypass, intank & pre-pump, reg return, Firestone air bags, Bilsteins, 350 HP/692 TQ
2000 Excursion- Pwrd by: T Wildman 6 pos chip, CT Performance Suspension, Riffraff Diesel Performance, J&M Tranny, 4" MBRP, 6637 filter, intank/prepump mods