Which Motor Oil Is Thickest?

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Introduction: Understanding Motor Oil Viscosity

The viscosity of motor oil refers to its resistance to flow. The American Petroleum Institute created a scale to measure motor oil viscosity. The recommended viscosity for your vehicle depends on the engine type and the outside temperature. It is vital to follow the recommendations in your vehicle’s owner’s manual to ensure optimal performance. In this article, we will explore the differences in thickness between various types of motor oil, including the thinnest oil option, engine oil viscosity, engine oil density, and other relevant topics.

An engine oils viscosity is displayed prominently on its container.

Which Engine Oil Is Thickest?

The thickest engine oil commonly available is 40W70 motor oil. This oil has a 40 cold temperature viscosity for cold starts and a 70 viscosity at operating temperatures. However, most modern cars do not require such thick oil. Antique or old vehicles may need extra-thick oil to prevent quick burn-off due to excessive oil consumption.

In summary, the thickest engine oil options are those with the highest viscosity ratings, denoted by the “W” rating.

What Does Engine Oil Viscosity Mean?

Viscosity plays a crucial role in determining engine oil thickness. The viscosity of motor oil is affected by operational viscosity (thickness at engine operating temperature) and the winter rating (W). Higher operational viscosities are suitable for higher temperatures, while lower W ratings perform better in cold conditions.

Viscosity measures a liquid's resistance to flow.

Viscosity represents a liquid’s resistance to deformation at a specific rate, reflecting its ability to flow. Engine oil products always have an operational viscosity rating, and some also have a winter rating. Low viscosity oil offers better fuel economy and performs well in cold weather. On the other hand, higher viscosity oil provides better lubrication at high temperatures.

Typically, viscosity is measured using units such as PI (poiseiulle), cST (centistokes), and 10-6 reyns (lb s/in2).

It is important to note that engine oil viscosity is not constant and can vary based on engine temperature, weather conditions, and usage.

Engine Oil Thickness Chart

Consider the following comparative densities (in kg/m3) and viscosities (in centistokes) of thick engine oils:

Engine Oil Thickness Chart

How To Tell The Thickness Of Motor Oil?

Determining the thickness of oil is straightforward as it is measured by viscosity. The viscosity rating consists of a number followed by a “W” and another number. For example, in the case of “15W-40” engine oil, the number after the dash (40) represents the viscosity when the engine is warm and running. The number before the dash (15) denotes the “Winter” or “W” rating, indicating engine performance in colder temperatures. Lower W numbers indicate better cold weather performance.

Comparing Oil Viscosity Ratings

Let’s compare some common oil viscosity ratings to understand their relative thickness:

  • 5W-30 and 10W-30: Both have a viscosity rating of 30, but 10W-30 is thicker at colder temperatures.
  • 5W-20 and 5W-30: At engine running temperatures, 5W-30 is thicker and has higher viscosity.
  • 10W-30 and 10W-40: 10W-40 has a higher viscosity rating, making it thicker at higher operating temperatures.
  • 5W-30 and 5W-40: While both have the same W rating, 5W-40 is slightly thicker due to its higher viscosity rating.
  • 10W-40 and 5W-30: 10W-40 is thicker in both operating and cold start-up conditions.

Comparing SAE Engine Oils

When dealing with SAE engine oils, we consider only one rating number, which represents operational viscosity. For example, SAE 40 is thicker than SAE 30 oil.

Which Motor Oil Is The Thinnest?

Understanding the comparative viscosities of engine oils allows us to determine the thinnest options. Regarding operational viscosity, SAE 16 is the thinnest single-grade motor oil. In contrast, the thinnest option for multi-grade motor oils is 0W-16, providing the lowest viscosity rating at operational engine temperatures. This oil performs exceptionally well in cold temperatures.


40W70 motor oil is the thickest option readily available. However, modern engines no longer require higher viscosity oils, as advancements in manufacturing have allowed for the use of lower viscosity options, providing better fuel economy. It is crucial to consult manufacturer recommendations and adhere to the appropriate viscosity for your vehicle.


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