In my experience, the high dilutions of Xtol starts to produce thin negatives when the stock-solution gets old but 1:1 dilution have worked well even with Xtol mixed one year ago (NOT kept in a refrigerator!). I store Xtol in full, dark, glass- bottles.(posted 8713 days ago)Kodak states that a minimum amount of 100 ml stock solution should be used for each film, with fresh developer. This means that a tank for 35 mm film that contains 250 ml of developer, cannot be used for the 1:2 dilution. I suspect that it will be even more critical if the stock solution is aging, although I have seen nothing in writing on this subject. A good habit would be to mix the stock solution in distilled water, since that water will not contain any oxidants (who would make the solution age faster).
From what I have read regarding the packaging, if there is no cake formed of the powder, it should be alright. If the powder have caked, don't even bother to mix it with water.