Had a choice of PEAK or "gumout". PEAK had a date code and I looked up on my phone how to read it. It was manufactured in April 2014. Couldn't find any date code on the gumout. I did once fill up at a bulk def pump. But I really wonder if they ever fully drain or syphon those out. They just keep adding new def to the old. My gauge is just above empty, but my CELL is on and I'm taking my truck in Wed. Code reader gives me a NOX sensor error & DEF regeneration frequency error code. But when I talked to the service guy (who isn't a mechanic), he asked me if my DEF was low. I said "Not that low that it would give me an error message". I was hoping to run this DEF tank almost completely out of reserve so I'm not adding new to old. My plan is to run it empty annually.