I don't know if it applies to all canadian Wal-Marts, but here's what's happening at my local one :
They are reorganizing the entire automotive section, including a major overhauling of the fuel/oil additives, oil & lubricants zone.
Looks like they won't carry Marvel Mystery Oil anymore as there was none to be seen and no shelf space labeled as such. If yours has some left, they may be on clearance.
They are also doing a clearance sale on Gumout Regane High Mileage at $5. At that price, I took the few bottles they had left.
Looks like the only place in Canada to get MMO now is Costco's online store.