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Good day all. I just traded up to a 2015 ram 2500 Laramie 6.7 cummins. I was browsing through the owners manual and it states to use 15w40 oil for temps above -18c and then use 5w40 synthetic oil for temps below -18c. My question is why conventional oil for warm temps and synthetic oil for colder temps? doesn't that defeat the purpose of using synthetic oil?:4-dontknow:
 

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Not a diesel dood, but I'm thinking you can use synthetic all year round. And, I'm thinking, you can use the 5w40 all year as well.
 

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15w is much thicker below freezing then 5w and that is why 5w is recommended for cold weather. However, since they are both 40 weight under normal operating temperatures I would suggest 5w40 all year long.
 
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