What you're doing 2.5 gals at a time is exactly what I did when all that was on my truck and it worked well, I never had a poor def fluid quality light. I also only used AC delco. My truck didn't get a whole lot of miles put on it on a regular basis, so that's the strategy I used. Wait for the warning message and add 2.5gals. I've seen a lot of dmaxes over in our Chevy shop with the def tanks being pumped out due to poor def quality, people constantly topping them off and or getting cheap def that's been sitting on the shelf too long. I know our chevy parts department purges the def every few months, if it's old it gets tossed. I'd keep doing what you're doing, like I said it worked well for me.
2011 BOWTIE LML ECSB SRW 4WD
Leveled, Debadged no moldings, CP3'd
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