All the Costco's in my area, greater Seattle, have 6qts of Mobil 1 5w-30 on sale for 26.99. It's normally 36.99. Not sure if this is nationally or just local. They have a limit of 3. They also have 10W-30.
This is the cheapest I've ever seen it. I usually go Walmart which has the 5qt jugs for 26.99.