oh man, don't use alkalines in anything you care to keep. do what i did, buy 16 rechargeables. i use NiMH energizers and haven't had any problems. they even survived my ignorance - i hadn't yet heard of "locking out the TK40". my light ended up turning on in a backpack on turbo. when i took the light out it was warm and the led was dimly glowing; just barely. it had been on for about 2 and a half hours in there. i still use that set of batteries.
i'm sure you can most likely use lithiums, i would just be careful to make sure you put them in correctly. i think a bunch of dummies put lithiums in the wrong way and bad things happened. maybe lithiums react much worse than alkalines when inserted improperly. fenix probably were just covering their collective behind when they made that statement. even though people made mistakes and blew up/damaged their lights, they most likely still called 7777 to see if they could get the light replaced; that may be what prompted the "new rule". this is just speculation on my part of course.
i just had to throw away my inova X1 because an alkaline was left in there and completely leaked and discharged all its inner goo, destroying the light. no biggie, i just kept that thing around for the aspherical lens.
...anyhoo, no alkies!