Amazon offers the Castrol GTX Magnatec 5W-30 5-Quart Full Synthetic Motor Oil Jug for $18.72. Check out via Subscribe & Save to cut it to $15.91 and net free shipping. That's $3 under what you'd pay in local stores, although we saw it for a buck less in July.

Note: Be sure to cancel your subscription after your order arrives to avoid being billed for subsequent shipments.