The purpose of this test, also known as the Ball Rust Test, is to determine how well an oil protects against rust formation after the engine is shut off. The need for this kind of protection might not seem obvious until you consider that water and acid are both byproducts of the combustion process. A certain amount of that water and acid ends up in your oil. So it’s important that some of your oil continues to coat critical engine parts, forming a protective rust barrier, even after your car has been shut down for the night and most of the oil has drained to the crankcase. Otherwise that water and acid could begin to rust your engine from the inside. In the Ball Rust Test a steel metal ball is submerged in the test oil. Acid is added to the oil. After 18 hours the ball is removed and inspected for signs of rust and discoloration. The used Quaker State® SAE 5W-30 that was tested after it had run for thousands of miles in New York City taxis was so durable that it still exceeded the ASTM D6557 standard for brand-new oil — even when it was ready to be replaced.