We usually get either a Fraser or Noble fir. I use straight water and make sure I keep it topped up every day. The tree is always green, fragrant, and pliable 3 weeks later when I haul it out of the house. Never had a need for additives.
The tree people said it's the recommendation from the American Association of Christmas Trees or something. The idea is that it help swell the "veins" of the tree, so it takes up water better. I'm afraid I just cooked my tree...
The aspirin thing seems to be an old wives tale. But at least the tree wouldn't get a headache.
Balsam or Fraser firs last the longest. Never have a needle drop when we get one of those. Fresh cut of course. The ones you get at the lots have been cut in early October. I have read of some tree cutters cutting them earlier and sinking them in swamps to preserve them until sale time.