Originally Posted by StevieC
Originally Posted by Munsy
New guy here with a 40W question inline with this thread. A lot of you would choose the 15w40 over the 5w40 if low starting temps was not an issue. I'm trying to figure out why that is? Just comparing Rotella T6 in 5w40 and their new 15w40, if it's summertime use and high temps are say around 100 F, is there a benefit to using the 15w40 oil? I was thinking both of these being 40w would be real similar for summer use. Am I missing something?
The benefit of using a 15w40 instead of a 0w40 is that the 15w40 uses less Viscosity Index Improvers (VII's) so it's less likely to shear. Ideally it's best to keep the gap between the winter rating (W) and the oils weight at operating temperature as close to each other as possible. EG: 15w40 is better than a 5w40 is better than a 0w40 where cold operating temperature permits.
Longer version of the story...
SAE30 and SAE40 were pretty well it...
The Engine manufacturers were dragged kicking and screaming into accepting multigrades, and 15W40 was the compromise (bar certain engines).
Later on, HTHS was researched as particularly the automotive multigrades were not protecting as well as their grades would have suggested...The high shear rate thinning was worked out, and HTHS became a new measure in the J300 tables.
The 0W, 5W, and 10W 40s were assigned the same minimum HTHS as the "30 grades", i.e. 2.9 minimum, which was reflective of the ability of the Viscosity Index Improvers at the time...wasn't a good thing, but it was what it was.
2013, the 40 grades were changed to 3.5 minimum HTHS for the 0W, 5W, and 10W grades, 3.7 for the 15W40s and up, still offering some delineation.
So one can't judge the decade old wider spread grades on historical issues.
If I had to choose an oil that would be compatible globally for every engine in every environment, a modern 0W40 A3/B4 API spec would do the job more than acceptably well.
If I had a narrower operating environment, a 15W30 would be my preference over a 15W40.
Just installed M1 Delvac MX 10W30 in wife's Captiva over the weekend...my Colorado, I'm stuck with 5Ws under warranty