For Prime members only, Amazon offers the Castrol GTX Magnatec 0W-20 Full Synthetic Motor Oil 5-Quart Bottle for $17.88 with free shipping. That's tied with our mention from last week, $16 off list, and the lowest price we could find. It keeps critical parts up to four times smoother.
Note: It's expected to ship within one to two months.
Update: A reader has noted that Walmart has it in [5W-20] for the same price via in-store pickup.