5w30 vs 10w30 – Differences in Engine Oil Viscosity

What’s the difference between 5w30 and 10w30 oil? These oils are different thicknesses in cold temperatures with 5w30 being the thinner of the two. For this reason, it’s recommended to use it instead of 10w30 for vehicles operating in cold climates.

In This Guide

The first number, 5 or 10 in this case, is a measurement of how easily the oil pours at low temperatures. The lower the number, the thinner it will be in these conditions: a 0 pours easier than a 5, a 5 easier than a 10, etc. An oil with a lower viscosity (a lower number) will reach and protect the engine’s internal components faster.

The second number, which is 30 for both of these oils, indicates the thickness of the oil once it reaches operating/high temperatures. A high viscosity (thicker) oil will adhere to mechanical components better when subjected to high pressure and stress and can withstand higher temperatures before thinning out beyond its working viscosity (past which it can no longer provide adequate protection).

When to Use 10w30 vs 5w30

The only difference between these two oils is cold flow ability: a 10w30 oil will move slower than a 5w30 oil during cold startups. At operating temperatures, both oils will have the same viscosity (30) and will flow and protect identically.

If the oil will be in the engine during winter and you live in a place where it gets cold during these months, use 5w30 oil. If it will only be in the engine during the summer, use 10w30. If you live in a tropical area where temperatures are consistently high and it never gets near freezing, an even higher starting viscosity oil like 15w40 can be used.

You should always use the weight of oil recommended by your vehicle’s manufacturer to provide your engine with the best protection and maximum fuel economy. Using an oil with a viscosity that is too high can result in excessive oil temperatures and increased drag, while using an oil with a viscosity that is too low can cause that oil to fly off of internal components when they’re in motion, leading to metal-to-metal contact that will accelerate engine wear. That said, most automakers will specify a range of oils that can be used in their motors, letting you choose the best viscosity to fit your driving conditions.

High Mileage Vehicles

In years past, owners of high mileage vehicles would use a higher viscosity oil than what was recommended by the manufacturer, say 15w40 instead of 5w30. As the oil passages wore down with age, switching to a thicker oil would increase the oil pressure, which would hopefully be enough to push it through every space in the block and heads to keep them lubricated.

Today, thanks to improvements in engine machining, oil formulations and filtration, wear on oil passages is negligible over the life of your car’s motor. That means switching to a higher viscosity oil can do more harm than good, putting more strain on the oil pump and preventing it from pumping it everywhere it needs to go. The manufacturer’s oil recommendations should be followed for the life of the engine; instead of using thicker oil when the motor starts to burn or leak oil, switch to a high mileage formulation. These oils address these issues with added conditioners that can help restore internal seals.

How Oil is Graded

Most oil produced by automotive oil manufacturers today is multigrade oil, designed to behave differently depending on the temperatures at startup and/or during operation. The viscosity numbers on the oil container are based on a test developed by the Society of Automotive Engineers (SAE) In this test, the viscosity determined by the number of seconds it takes for the oil to flow through a specific size tube.

The first viscosity number is the “winter” number, hence the “w” after it. This number is tested at 0°F. The second number is tested at 212°F to simulate the heat of an engine at operating temperatures. That means a 10w30 oil takes 10 seconds to flow through the tube when cold, and 30 seconds when hot.

Normally, fluids thin as they warm up and thicken as they cool, but these oils have additives that cause the opposite to happen.

Single Grade Oil

Before thickening additives were developed, oils were only available in a single grade, which would have to be changed to work at seasonal temperatures: a heavy oil would protect well in summer, but a light oil was needed in winter so it would pump through the engine when cold. Since single grade oils have one viscosity, they’re labeled with only one number, like SAE 30.

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Over to You

We’re interested to know – which of these oil ratings did you use in your vehicle and what was the temperature outside at the time? Let other readers know by leaving a comment below!

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