2018 Duramax Fuel Mileage

Every couple of weeks my fuel mileage drastically goes down for a day or so then goes back up. ( Looking at the instant mileage screen). Is this caused by the exhaust regen? Should I get a message when regen starts, seems like I did once. Also would brand of DEF have any effect? There sure is a large range of price

I wouldn't even bother looking at the instant mileage screen. It is nothing more than a guess based on calculations of conditions. As long as your actual overall mileage isn't dipping then you don't have an issue.

And only one way to check the mileage. Fill it up, zero out odometer. Drive it. Fill it back up. Divide mileage by how many gallons it took.