Yea, I buy mine at Walmart too. Never had a problem before. I do avoid Fram filters for the Miata, though.
If you're getting a coupon for a free jug, head over to a parts store and pick it up there. Maybe that way, you'd probably be picking from a different "batch" that might be good. Explain your story and I bet they'd even let you open it in the store before you leave.
It wouldn't be the first time wal-mart knowingly accepted lower quality goods at a discount. It also happens with tires.
Something interesting, the Castrol GTX 5 qt jugs are labeled specifically for Wal-mart, they have something about recycling the oil there.