You probably know all of this, but I'll mention it just because it's worth mention.
I would NOT stake a once in a liftetime trip on unknown expired film. Most of my 4x5 Velvia 50 has been bought in largish lots(and I keep them bundled by lot from the same source) so I can be reasonably confident it all will behave the same. Still, if I were flying to the other side of the world and wanted to shoot Velvia 50(which I would), I'd pony up the money to get fresh stock.
With B&W, you generally are just dealing with decreased sensitivity and increased base fog. With color, you have to factor in color shifts, and this is another area where reversal film is unforgiving. If you're going to shoot expired film, try to find a freezer clean-out on Ebay, buy as much as you can get from the same source, and test it to see how it behaves.
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