Many circulation and oiling issues of the past have been solved, along with tighter tolerances, so switching to extreme weights of oil is not always necessary, and could actually cause your engine to perform worse. Fast forward to today’s engines, and it’s not uncommon to see engines on the road with 300-400,000 miles on them. It’s important to note, that your engine’s oil pump, and thus the engine, has to work extremely hard to pump thicker oil and maintain oil pressure, so folks that would resort to 20w50 oil on their tired, old engine would also sacrifice horsepower using it. An engine’s life began to deteriorate rather rapidly around 100,000 miles, and so to increase the oil pressure, high-mileage car owners would use a higher oil viscosity such as 10w40 or 20w50. Like mentioned before, in the past, engines were built with looser tolerances – especially bearing clearances. A majority of manufacturers are now producing engines that operate on 5w20 oil, to which it’s important to stick to this viscosity and not substitute 5w30. So, due to viscosity, 5w20 is a thinner oil during operating temperatures, whereas 5w30 is thicker during operating temperatures. This enables the 5w20 motor oil to reach engine parts quicker and create less drag that reduces fuel economy. 5w30 motor oil, the 20 indicates that the oil has a lower viscosity and is thinner at higher temperatures. The viscosity number is the result of the Society of Automotive Engineers (SAE) determining how fast the oil will flow through a certain size tube.

This number is tested at 212 degrees Fahrenheit. This indicates that the oil is thicker at higher temperatures, so it adheres to the metal parts of the engine, protecting them as they operate. The 30 refers to the viscosity at higher temperatures. The viscosity of the winter number is tested at 0 degrees Fahrenheit. The “w” behind the five and the 10 indicates that those are the viscosities in colder or winter temperatures. 10w30, the thinner 5w30 will circulate quicker, and flow through bearing tolerances better during an engine startup in the winter or colder climates. So, that means when you are comparing 5w30 vs. The five and 10 digits refer to the viscosity at cooler temperatures. What’s vitally important, is that you stick with what the manufacturer recommends for oil viscosity. Viscosity will determine how your engine’s oil will deal with changes in temperature, pressure, and speed. A thinner oil reaches the engine parts quicker, and circulates faster, while a thicker oil tends to adhere more easily to the engine parts, and will provide better lubrication when engine tolerances have deteriorated, or in severe-duty applications like racing or off-road engines. More recent engines have been built with better technology, and much more strict tolerances, which also means a much thinner oil is necessary to properly lube the engine’s bearings, and circulate quicker and freer throughout the engine, and often times with less overall oil in the system. Older engines were built with much looser tolerances, which allowed oils such as 10W30 to be commonplace as an OE recommended oil in the 1990’s and before. Where this is vitally important is in an engine’s bearing clearances.

A higher oil viscosity number indicates a thicker oil. The term “viscosity” refers to the thickness of the oil and its resistance to flow.

Oil viscosity