Where's the old man on this one?
Long story short, dad picked up a bunch of still-sealed Pre-war Navy dope back in the early 90s. He got them just for the collectability of the cans, which were in good shape, but after ALOT of stirring, found that the dope inside was still viable. He experimented with thinners (DuPont 3608S if I remember right) and used it to paint several models over the years. This is an extreme example of course, but the moral is, as long as it has been sealed properly, and tests are producing good results, it ought to be just fine.