Is 5w40 really worth it for cold climates?

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I logged over 275,00 miles on my 2005 (LLY) Duramax using Shell Rotella T3 (now T4) with very good UOAs. I live in MN and would often see cold starts in the -10 to +20* F range. Thankfully I usually parked in a attached garage which helped with the morning start, but the afternoon one was always at ambient. I never really noticed a hard starting condition, but I will say I did frequently see oil pressures in the 100+ PSI range. I also took care to not work the truck hard or fast until things got a little warmed up, but I am also not one to sit and idle - that just wastes fuel.

I don't really care about the synthetic vs. conventional debate. If there was a dino oil that was 5w40, I'd already be using it. The only reason I am considering changing to 5w40 is for cold starting and emissions systems. I recently sold my 2005 and now have an 2018 (L5P) which is fully emissions compliant. I am thinking using a thinner oil will aid in keeping the DPF cleaner longer during winter time. I haven't seen much data around this topic with an emphasis on cold starting and emissions. I am not worried about wear metals or extended drain intervals, with the Duramax it simply doesn't matter.

Does anyone have any first hand experiences with switching to 5w40 purely for emissions or cold starting?

I am hoping @dnewton3 is able to chime in on this one too. He has more experience with this engine and different oil viscosities than anyone.
 
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A UOA isn't going to show a thing regarding cold starts. If you are below 0F then an oil with a winter rating of 5W will be somewhat thinner than an oil with a 10W rating, so yes the engine can crank easier. But this is also somewhat mitigated with a good battery.

I don't know what you mean by emissions and the winter rating. The engine and the oil will warm up faster with a thicker oil.
 

nautique

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Correct. I am not looking for UOA data.
My ask is mostly regarding emissions. Soot content and ash content the oil may produce.

Let me rephrase. Do thinner synthetic oils produce less soot than thicker conventional oils under non-ideal operating conditions?

EDIT: To further rephrase because I re-read my original post and can see the confusion.
Is there a benefit to the diesel emissions systems to run a 5w40 synthetic oil? If there is no emissions benefit I see no need to run synthetic unless it's for cold starting, which I haven't found to be an issue personally.
 
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Does an engine produce more soot when cold? Then as I mentioned a thicker oil will cause the engine to warm up faster.

But this would only apply to the very early portion of the warm-up phase, once the oil has reached a temperature around freezing then any winter rating (and any grade for that matter) will be much the same up to the operating temperature. One would be hard pressed to try and quantify and measure the very tiny amount of time where the oil was significantly thinner because of a lower winter rating.

And it really also doesn't have much to do with whether it is a synthetic or conventional, it would have everything to do with the grade.
 
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At about $21/gallon for 5W40 synthetic (Shell T6) I am not sure how much you could save if you found a non synthetic 5w40. Maybe $5 a gallon tops.

Plus you can go more miles per OCI using synthetic. I have all the emissions stuff on my 2015 and go over 20K miles per OCI. (I do have a bypass filter however).
 

nautique

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I can get a 2.5 gallon jug of T4 for $32 bucks. I bought my B1441 filters in a box of 10 for $8.43 each. $40.43 for 10k changes it good with me.

I don't know if an engine produces more emissions (soot) when cold. I'd assume it does, as combustion isn't as efficient. My trucks usually take 10-20 minutes of drive time before the thermostat opens, around 10 minutes for the oil pressure to become normal. When you're doing lots of short trips in the cold, you may never reach the efficient mark - hence my question. Then the factor of syntactic vs conventional with how efficient they burn. Which isn't a factor of grade, but oil makeup.

I get the easy answer is to just run synthetic and shut up. It's a win-win. However, I'd also enjoy understanding if there are other factors to make the switch.

Sounds like there isn't much data on the subject. I guess I will start logging my oil consumption, fuel consumption and miles between regens and run a while on dino and then switch to syn and run a while. Worst case I learn something new.
 
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FWIW I've been using Rotella T4 15w40 in my '17 RAM. Pretty much all it does is pull a heavy 5th wheel, short drives to the store, or plowing the driveway in winter. I change the oil based on time for warranty purposes. No emissions, starting, or driveability issues to report in almost 4 years.

Cold start temps are generally in the same range as yours.
 
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Living in Michigan I've noticed easier cold starts with 5W40 T6 when not plugged-in when its around 0-20 degrees outside with the truck. Even more so with my older 3-cylinder Yanmar John Deere tractor.
 
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Here is the owners manual from a 2015 Duramax. Below 0 degrees F you should be running 5w40. Does it help with the cold start? YES.
558B9796-C988-488C-BD48-59EAE74E2A99.png
 
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I logged over 275,00 miles on my 2005 (LLY) Duramax using Shell Rotella T3 (now T4) with very good UOAs. I live in MN and would often see cold starts in the -10 to +20* F range. Thankfully I usually parked in a attached garage which helped with the morning start, but the afternoon one was always at ambient. I never really noticed a hard starting condition, but I will say I did frequently see oil pressures in the 100+ PSI range. I also took care to not work the truck hard or fast until things got a little warmed up, but I am also not one to sit and idle - that just wastes fuel.

I don't really care about the synthetic vs. conventional debate. If there was a dino oil that was 5w40, I'd already be using it. The only reason I am considering changing to 5w40 is for cold starting and emissions systems. I recently sold my 2005 and now have an 2018 (L5P) which is fully emissions compliant. I am thinking using a thinner oil will aid in keeping the DPF cleaner longer during winter time. I haven't seen much data around this topic with an emphasis on cold starting and emissions. I am not worried about wear metals or extended drain intervals, with the Duramax it simply doesn't matter.

Does anyone have any first hand experiences with switching to 5w40 purely for emissions or cold starting?

I am hoping @dnewton3 is able to chime in on this one too. He has more experience with this engine and different oil viscosities than anyone.

I live in eastern Iowa and use Mobil Delvac 15W40 in my truck. When it gets down close to 0F it does start a little slower than usual. I don't drive my truck below 0F because you can't find #1 Diesel around here and I don't need the headache I had in 2019 of gelling up...

As far as emissions I don't think oil type will play a part in that.

Just my $0.02
 
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15W generally means -20C is acceptable, 10W generally is good to -25C, 5W to -30C, and 0W to -35C.
 
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15W generally means -20C is acceptable, 10W generally is good to -25C, 5W to -30C, and 0W to -35C.

For sure. Those numbers are from the cold cranking data. However, for diesels, I would recommend following GM’s recommendations and run 15w40 to only 0 F (-18 C). Heck, a lot of folks plug in their diesels at -20 C.(-4 F). It makes more a much smoother start with no “chugging”.
 
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If you research pour points and CCS values, Rotella T5 10W30 (synblend) and T6 5W40 (full syn) are very close. Not sure if 10W30 is allowed in a Duramax, but T5 has proven to be a good choice, and is usually cheaper.
 
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Bullwinkle, that's a good point that has been brought up before. There are some 10w30 oils that while are rated 10w and tested at the- 25c, have cold flow very close to what 5w40 does.

I ran/currently run PetroCan duron 10w30 in my Cummins. It's cold flow is very good and its numbers are not far off a 5w40. At over 2$ a litre cheaper than its 5w40 counterpart, its CES20086 approved. It's cold flow is better than T5, Citgo and premium blue. All the while cheaper as well.

I found no issues running this for the last 3 years. None perceived and nothing showing on my UOA. Because of this, my next change will be going to a locally made group 2 10w30 CK4. One that slightly betters T5 cold flow.

As well i think there is plenty of UOA and discussion on this forum citing that a 10w30 is more than adequate in any of the Duramax lines.
 
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For sure. Those numbers are from the cold cranking data. However, for diesels, I would recommend following GM’s recommendations and run 15w40 to only 0 F (-18 C). Heck, a lot of folks plug in their diesels at -20 C.(-4 F). It makes more a much smoother start with no “chugging”.

I think we are saying the same thing, within 2 degrees.
 
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FWIW Cummins specs 5w-40 in the 2019 6.7 if 0°F/-17.77C or colder. In short it seems it's required by at least two manufacturers.
 
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I switched to T-6 5W40 in both of my Mitsubishi medium box trucks. I logged almost 300,000 miles on them. When temps were around 0 they would start right up. Never had to use a heater in the winter, When other diesels weren't starting from the temps, mine were always up and running.
 
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FWIW Cummins specs 5w-40 in the 2019 6.7 if 0°F/-17.77C or colder. In short it seems it's required by at least two manufacturers.

They recommend it for the '17, too.

I've been caught out a couple times, where I started it at a few degrees below 0° F, without having the block heater plugged in. It started fine, oil pressure came right up, and it ran fine. If I were doing that regularly, I'd switch to a synthetic 5w40 they recommend. If it were really cold, say -40°, I'd be running the lower viscosity synthetic for sure.
 
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