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My 2013 3500 SRW Aisin has had synthetic 5w40 since the first change and now has 70K miles. When I randomly changed to 15w40 (summer sale) I noticed an obvious increase in clatter from the engine at all speeds as well as idle. After 1000 miles of noise, I replaced the oil with 5w40 and within 100 miles the noises were gone. What would cause the increase in engine clatter. I really wanted to run 15w40 for summer but am hesitant to try again. FWIW, the 5w40 is Valvoline Synthetic and the 15w40 was the new T6.
 

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At operating temps 5W40 and 15W40 are same weight (40). The 5W gives you better cold start protection. I use 5W40 year round. Now that I’m moving to MO where it does get colder I’ll switch to 0W40 and use it year round.

Not sure what would be causing the noise. Albeit it’s different brands; so my guess is it’s related to the brand and not 5 vs 15 for the cold viscosity.

I’m not familiar so maybe T6 is valvoline...if so disregard


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Yes, The viscosities at oper temp are the same so it didn't make much sense to me either. I did use different brands of oil but that shouldn't really matter, right? T6 is Rotella 15w40 synthetic and 5w40 is Valvoline synthetic diesel. The excess noise was obvious throughout the rev range but more so at lower rpms.
 

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Yes, The viscosities at oper temp are the same so it didn't make much sense to me either. I did use different brands of oil but that shouldn't really matter, right? T6 is Rotella 15w40 synthetic and 5w40 is Valvoline synthetic diesel. The excess noise was obvious throughout the rev range but more so at lower rpms.
My only point was no need to change to 15w40 based on time of year. If anything 5W-40 gives you better cold weather performance; while still providing 40 weight oil at operating temp. Yea no idea why there would be noise. I was just speculating that maybe it’s something in the formulation; would be even stranger if it was the same brand and the change produced the noise.


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Splitting hairs, it's possible that the oils were on opposite ends of the 40 weight viscosity bracket and had a few cSt difference at operating temp but in reality, this engine should not even care. Hell, mine ticks along smooth as it ever did on a 12 cSt 30 weight.
 

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This does not sound promising. I am changing my oil today from 5w40 T6 to the 15w40 T6. Why would it take 100 miles for the sound to go away once you swapped the oil out? It should have been gone immediately if it was an oil issue. Could you have had some bad fuel? Bad fuel is the only thing i have ever had change the way a Cummins sounds.
 

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Just run the 5w40 year round.
 

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Just dumped in three gallons of the 15w40 T6. Truck sounds the exact same as with the 5w40 T6.
 

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I run 5W40 year round in Michigan, running 7.5K to 10K changes without issue. It gets hot in the summer 90+ and cold during the winter. (-19F) was the lowest I ever started up without being plugged in. UOA's come back looking good too.
 
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