Cummins specs 5w40 and 10w30 in their new version of the 6.7 engine. Not to use 15w40 due to the hydraulic rollers on their lifters. My question is why 15w40 will damage the lifters and 5w40 will not if the oil is the same thickness at operating temperature? Or is 5w40 thinner at operating temperature and just has the protection of 15w40 weight oil?
newer Cummins 6.7L ... 5w40 vs 15w40
- Thread starter NiwradND
- Start date
Propflux01
$100 Site Donor 2026
The 15W will not flow as quickly in colder temp start up than the 5W
Than why the blanket statement it’s never allowed such as in Phoenix AZ in 115F temps.
I run 5w40 year around, so if your statement is true, inwhich I believe it is, we should be able to run 15w40 from 40 degrees on up to 122 degrees and maybe we should have 0w40 in extreme cold. At -30 mine still sounds rough at start even when plugged in.
Yes 5w40 is good year around
Propflux01
$100 Site Donor 2026
What blanket statement? I stated no such thing. The OP stated "Not to use 15w40 due to the hydraulic rollers on their lifters". The 40W part is not the issue, it's the 15W part. Apparently Cummins thinks it is too thick to use. 15W is thicker than the 10W or 5W, and that may be an issue with the first few moments of a cold start on these engines.
Last edited:
Well the reading comprehension & attention span of the public is worse than my Irish Setter mix. Way easier and cheaper from a liability standpoint to say "never, ever" instead of "well maybe if it is warm in AZ".
I was not referring to you I guess as more to the owners manual
Agree me thinking just say no to any 15w because there are many bad ones on the market for various reasons.
This subject has been covered over and over with no flat out answer ever.
https://static.nhtsa.gov/odi/tsbs/2023/MC-10242795-9999.pdf
The second page says 2019 or later require 10w30 enjoy if you haven't read.
The second page says 2019 or later require 10w30 enjoy if you haven't read.
Read entire thread please, 15w40 is discussed multiple times.
Yes, It says 10w30 or 5w40. 5w40 year around also works, I think 5w40 is better on both ends of extreme temperatures.
I concur, I'd run a 5W-40 all year round. I personally wouldn't trust a 30 in such a torquey diesel.
Yes I run HPL 5w40 all year in my 21 HO 6.7l in Phoenix AZ but the vehicle could come north in a Wisconsin winter.
Because I don’t feel like searching for more posts about this on Cummins forum or HD rams forum; which there are plenty….
I'm not convinced this a accurate statement, but thats just me.
5w-40 is going to flow to the hydraulic lifters sooner than a 15w40 at very cold temperatures. They must have had some issues regarding extra deposits due to using 15w-xx in cold climates. If those deposits potentially caused issues with the lifters that would not be good. As a result they're recommending a 10w30 or 5w-40 depending on the temperature. If you don't want to change between two different grades of oils throughout the year you can safely run 5w-xx that is approved for or meets the spec.
Deposits.
FCA thinks running Full Synthetic will prevent the deposits formed when someone runs bargain basement conventional 15w40 at their nonsense 15k OCIs.
They don't want you running conventional so they blanket banned all 15w40. All the punters think it's because it's related to flow. It's not.
Pick a 5w-40 and stick with it. Delo, Delvac, Rotella, even Fram, etc.
FCA thinks running Full Synthetic will prevent the deposits formed when someone runs bargain basement conventional 15w40 at their nonsense 15k OCIs.
They don't want you running conventional so they blanket banned all 15w40. All the punters think it's because it's related to flow. It's not.
Pick a 5w-40 and stick with it. Delo, Delvac, Rotella, even Fram, etc.
Last edited: