Go to Walmart and buy a 5 quart jug of Valvoline MaxLife High Mileage motor oil for $16.97. The weights available include 5W-20, 5W-30, 10W-30 and 10W-40
From there, go here and get $10 off. No mail-in rebate required.
Finally, if you use the Ibotta app, you can get an extra $3 off.
Your total will come out to $3.97 along with whatever sales tax you pay on the initial $16.97. For most folks that will equal an overall purchase price of less than five bucks for a product that typically sells for between $20 to $30.
Want to have it delivered to your door instead? If you have Amazon Prime then it's only three dollars more. Go here and then here. Right now you can also opt for some healthy rebates (here, here, and here). Although the Valvoline remains the cheapest if you plan on following the usual 5,000 to 10,000 mile oil change regimen that most manufacturers recommend.