Amazon offers Prime members the Mobil 1 5W-20 Synthetic Motor Oil 1-Quart 6-Pack for $26.99 with free shipping. (Non-members can get free shipping with orders of $49 or more; otherwise, shipping adds about $10.) That's the lowest total price we could find by $9.

Also, Auto Parts Champ via Amazon offers Mobil Super 5W-30 Motor Oil 1-Quart 6-Pack for $27.50 with the above shipping options. That's another price low, this time by $23.