Walmart offers the Castrol GTX Magnatec 0W-20 Full Synthetic Motor Oil 5-Quart Bottle for $17.88. Choose in-store pickup to dodge the $5.99 shipping fee. (Amazon charges the same price, for Prime members only.) That's $16 off list and the lowest price we could find. It keeps critical parts up to four times smoother.

Update: It's now sold out at Walmart, but Amazon continues to offer it exclusively to Prime members for $17.88 with free shipping.