20W-50 vs 5W-20: Which Oil Protects Your Car Engine Better?

“To get real results, we need a real engine,” states Lake Speed, a professional oil expert and host of The Motor Oil Geek YouTube channel as he puts two identical motor oils to the test with the only difference between them is their viscosity: one oil (20W-50) which is thicker than the other oil (5W-20) commonly used in many vehicles today.

What is Motor Oil Viscosity?

Viscosity in your car’s motor oil refers to the measure of the oil's resistance to flow―a crucial property because it affects the oil's ability to lubricate engine components under various conditions.

The primary factor of a cold vs a running engine and its effect on viscosity is that as the temperature increases, motor oil becomes thinner (i.e., less viscous), and as the temperature decreases, it becomes thicker (i.e., more viscous).

Your Motor Oil Viscosity Grades

Motor oils are classified by their viscosity grades, as determined by the Society of Automotive Engineers (SAE) and are typically represented by numbers and letters such as "5W-30."

These grades consist of two parts related to both cold and hot engine running conditions:

• Low-temperature (“Winter” NOT “Weight”) Viscosity: Indicated by the number before the "W" (e.g., 5W). This number signifies the oil's viscosity at cold temperatures, ensuring that the oil will flow and protect engine parts during cold starts.
• High-temperature Viscosity: Indicated by the number after the "W" (e.g., 30). This number represents the oil's viscosity at the engine's normal operating temperature, ensuring proper lubrication and protection under high heat and stress.

Other Grade Classifications in Motor Oils

Modern motor oils are often multi-grade, meaning they are formulated to perform well across a wide range of temperatures. For instance, a 5W-30 oil will behave like an SAE 5 oil in cold temperatures (providing easier starting and better flow) and will behave like an SAE 30 oil at high temperatures (providing adequate lubrication and protection).

For example, if you buy single grade motor oil for your lawn mower, typically it says “SAE 30” on the bottle because you only need an oil for warm or hot summer use and not winter use when temperatures are much cooler.

Why Your Motor Oil Viscosity is So Important

The importance of your motor oil’s viscosity is that it is the foundation of several factors when it comes to your car’s engine that includes:

Lubrication: Ensuring that the oil forms a protective film between an engine’s moving parts to reduce friction and wear.

Cooling: The viscosity affects the oil's ability to carry excessive heat away from engine components where friction and wear are occurring.

Fuel Efficiency: Viscosity can improve fuel efficiency by reducing the energy required to overcome friction. For example, the harder it is for a piston to rub against the cylinders, the more fuel it takes to power the pistons.

Protection: Viscosity helps maintain engine cleanliness by suspending contaminants and preventing sludge formation through detergents and other additives used in motor oil formulations.

The Most Important Factor You Need to Understand About Motor Oil Viscosity

However, the most important factor when choosing the right viscosity grade for your vehicle is making sure you are using the manufacturer’s recommendations. Based on the design of the engine, oil flow passages and galleys vary in size and routing within an engine requiring that only a specific viscosity will adequately squeeze through and adequately lubricate between the surfaces of moving parts that possess different clearance tolerances.

In other words, if an oil is too thick it will not get into tight spaces between moving parts like bearings and shafts for proper lubrication. If the oil is too thin, the moving parts will also not get enough lubrication because the oil is running too thin between the moving part surfaces.

20W-50: Will This Thicker Oil Protect a Test Engine Better Than 5W-20?

To determine which viscosity of motor oil offers better protection, the host of The Motor Oil Geek YouTube channel visits a testing facility where a specially designed race engine on a dyno is the test equipment used that not only measures the horsepower of the engine, but also sensors to measure compression within the cylinders to see how a difference in viscosity affects both engine performance and engine wear.

Related article: The Truth About Whether Valvoline’s Restore & Protect Motor Oil Really Works

In other words, to determine how using motor oil with two different viscosities affects their measurements on the dyno.

Follow along with the host and the dyno tests to find out how motor oil viscosity affects both a cold and hot engine that includes some interesting info such as:

• Why leaded fuel vs non-leaded fuel will throw off your bearing wear oil analysis.
• Whether pre-filling an oil filter with motor oil makes a difference.
• What happens when you do not prefill an oil filter and how this differs between different oil viscosities.
• How long it takes for an engine to reach its operational oil pressure from a cold start with both thin and thicker viscosity motor oil.
• Which oil viscosity results in the most amount of damage to an engine during both the start-up times when the engine is cold and when an engine is hot during a hard run?
• How much each oil changes its viscosity between room temp and fridge temp conditions.
• How power is affected when oil is added to the point of being overfilled within the engine.
• How oil viscosity affects windage with oil flow in an engine.
• Which oil resulted in more wear and where the wear occurred.

Related article: What Happens When You Shake Old and New Motor Oil

20W-50 vs 5W-20: Which Oil Protects Better?

Two Main Takeaway Findings:

Bear in mind the tests were done on a race style designed test engine with greater clearances between moving parts compared to the tighter clearances you would expect in a normal car sold today for non-race conditions. However, the results from whether to prefill your car filter with oil (or not) with a thinner manufacturer recommended motor oil appears to be useful and applicable advice for car owners.

1. Increased viscosity with the thicker 20W-50 motor oil―especially when overfilling to keep oil pressure high―resulted in decreased horsepower as expected.
2. Not prefilling the oil filter with the lower viscosity 5W-20 motor oil resulted in more wear than if you did prefill the oil filter with the same oil. Furthermore, prefilling and not prefiling the oil filter using the higher viscosity motor oil showed no difference in wear, which was not expected.

For additional engine oil related articles, here are three selected articles for your consideration:

• The Truth About Motor Oil Additives
• How Much Oil is Too Much Oil in Your Performance Car?
• Why You Should Not Trust This Motor Oil for More Than 5,000 Miles

Timothy Boyer is an automotive reporter based in Cincinnati. Experienced with early car restorations, he regularly restores older vehicles with engine modifications for improved performance. Follow Tim on Twitter at @TimBoyerWrites  and Facebook for daily news and topics related to new and used cars and trucks.

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