I'm that guys who bought all that Sears Ultra Plus Detergent.

This is what I have read and understood-

"Powdered Formula Laundry Detergents

Powdered laundry detergents do not expire or lose effectiveness unless they have been exposed to moisture.

If powdered detergent becomes hard or cakey, it should be discarded. The powder will probably not dissolve correctly or completely in the wash leaving soap deposits on your clothing. This can easily happen with a homemade powdered detergent because it does not contain anti-caking ingredients."

Mine is sitting in my dry cellar which stays around 65-70 degrees year-round.

Jeff

The oxygen bleach component of Oxi-Clean will lose effectiveness if it is exposed to moisture. Clumping is usually a sign of moisture exposure. You can test to see if it's still effective. Mix with very hot water, stir to dissolve. Bubbles should form. Dip a paper towel or white rag in some non-greasy organic source of stain, like beets, ketchup, red wine, etc. How long does it take to remove the stain? If it has reduced effectiveness, you can give it more time, more heat, and/or raise the pH with baking soda (be careful because higher pH can damage some fabrics) to increase its effectiveness. Also, oxygen bleach even when fresh works better when given plenty of time. At least half an hour, or overnight soaking. If old, clumped doesn't remove a non-greasy organic stain within an hour, I would toss it.

I have kept Oxi Clean bought at Costco working for years by transferring it from the cardboard box into a tight closing plastic bucket. I am careful to keep it dry when scooping out and close promptly.