I was wondering what people's habits are when adding DEF fluid. This is my first diesel that I have had to add DEF and with 15,000 miles I've only filled it up a few times. Got the message again today that it is low, but the previous times I let it go till the warning of speed limitations came on then added. My thinking on this is that since DEF does have a shelf life if I pretty much ran the system out I would have all fresh in compared adding a 2.5 every few thousand or so. Is there any harm in running the tank dry? I know you really don't want to run these newer diesels low on fuel because of potential increased contamination in a low tank. The times I have filled up it has taken five gallons. I have been going to the dealer and getting the AC Delco stuff just in case I ever had an issue. It's more expensive but who cares if it's $5 more I guess. This is my farm pickup so I almost never get 50 miles away from my house so it would be unlikely I'd ever get in a situation where it would actually be speed limited before I could get DEF.