Paul, what watch did you wear at Oil Creek? I had a garmin 910xt that I charged on the run and a garmin 920xt on ultratrac mode. The 920xt died at about 23 hours and wasn't real accurate as I expected in ultratrac. My 910xt wasn't as accurate as it has been. Your results seem much more accurate.
Andy, it's a 910XT. I charged it on the run after 2 loops. However at some point I noticed my last mile pace was blanked out and I realized the timer was stopped, so I restarted it. So I'm not sure how much of a dead spot I have. Hopefully not much.
That's one criticism I have of the watch. The buttons for start/stop are too sensitive and they tend to get triggered when I reach for something in my pack!
I think my 910xt logged 95 miles so I guess that's good for the terrain. I agree the buttons are sensitive. I realized my 920xt was not in fact on ultratrac but smart recording. The weird thing is it kept showing miles but the timer stopped at 14 hours. I'm thinking about getting a suunto for 100 milers.