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Chevrolet recommends AC Delco Diesel Exhaust Fluid in 2021 Sliverado HD Duramax. Is their a difference in AC Vs Blue? Most  Stores don't carry AC brand.. is it worth ordering online or is any DEF fluid okay? 

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Any DEF will work fine.  The main thing is to get it as fresh as you can. It does have a shelf life.  If you have a truck stop that has it at the pump that is the best, fresh, kept underground so it is not in heated area and is the least expensive.

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Many many threads on this subject on the sites that focus on the 6.6


Bottom line is use whatever you want.


I'm currently using bulk from a truck stop pump, but that is not convenient for everyone. 

Before we moved, this was not convenient so was a 2.5gal jug user for many years. 


Only ACDelco branded used in any of mine was what may have been it it when delivered new. 


Peak now has two versions.

Save your money and use the original Peak Blue if you decide to use that brand.


Manual says:

Use only DEF that is GM approved, or fluid containing the
API certified or ISO 22241 label.
The use of other fluids could damage the system, requiring
costly repairs that will not be covered by the vehicle warranty. 


You are going to find that all brands will have ISO 22241 on the label.


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