Engine oil prices vary depending on the vendor and type of oil. The most affordable type of oil is conventional oil, often used in bulk at dealerships. Popular engine oil brands include Mobil, Valvoline and Castrol. As of October 2015, the cheapest brands on Amazon.com are Pennzoil, Peak and Delo.
A vehicle's owner's manual recommends the ideal type of engine oil and viscosity for the vehicle. Conventional oil is ideal for low-mileage engines that require frequent oil changes. Popular brands such as Pennzoil offer many types of engine oils, such as high-mileage, conventional and synthetic. Pennzoil claims to clean out up to 40 percent of engine sludge after one use.