My understanding is that this is just based on a desire to have the DEF as fresh as possible. If you wait til you get the warning, it'll be mostly newDEF in the tank. I have a Chevy Duramax and add a 2.5 gallon jug the next chance I get after the warning. Its usually good for another couple thousand miles then, even towing.
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2016 Montana 3160RL w/MOR/Ryde IS and Disc Brakes
855W Solar System, TriStar MPPT-60 Controller, Magnum 2800W Inverter, 6 Trojan T-105REs
2015 Chevy Duramax SRW SB Crew Cab
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