For Prime members only, Amazon offers the Rotella T Triple Protection CJ-4 15W-40 1-Gallon Motor Oil, model no. 550019913, for $12.97. Order via Subscribe & Save to cut it to $11.02 and bag free shipping. That's $3 under what you'd pay in-store locally.
Be sure to cancel Subscribe & Save after your order ships to avoid being billed for subsequent shipments.