I wouldn't trust the old adhesive, ask Dorsai about his tube of RTV.
I used 3M emblem adhesive on my old F150's Lariat and Trailer Special emblems. It set up nicely and did a really good job at keeping the emblems secure. I can't remember which one it was, but I picked it up at the local auto parts store in the auto body section. When I sold the truck a few years later I thought about taking the emblems off but they wouldn't budge without some work so I left them.
Whatever you do, don't use liquid nails like the previous owner of my current truck did. I'm gonna have to sand down and repaint the emblem area on my truck since chipping off the liquid nails took the paint with it.