I found an unopened 4L tin of cheapy gloss white under a bench in the workshop and decided to use it. Bit of a laugh, it said, 'Do not stir' - as it was supposedly a jelly type. When I opened it I found a couple of inches of pigment in the bottom and on top of that was several inches of brown fluid. Stirring away for a good few minutes yielded an ok white paint, but I wondered if it would still be any good.
That was yesterday; today the first coat has dried fine, and since it's only a workbench I'm painting I don't know if I cba doing a second coat.
The tin is twelve years old, if it's of any interest.