A thicker oil may stop engine knocking, depending on the cause of the knocking and how much damage has been done to the engine. Older engines often run better on slightly heavier oil that runs thicker when the engine is fully heated. This is because they have more internal wear than lower mileage engines and thicker oil will increase the oil pressure allowing for better lubrication. A thicker oil may make engine knocking worse if it means that the oil does not flow as freely around the engine. The latest engines that use fully synthetic oil run with very specific oil specifications due to emissions restrictions. Using thicker oil may actually damage these engines and cause premature wear along with a decrease in fuel economy.