This is one of those topics that comes up from time-to-time...it would be nice if we could have it "pinned" to it would stay on top.
I was facing this same issue (Flexament getting too thick) several weeks ago. The local Sportsman's Warehouse USED to have the Flexament Thinner. But it doesn't have any now. I grabbed their last bottle of Griffin brand thinner, though. It worked just fine.
Good to know about the Toluene or Xylene and the Shoe Goo, too!