ENGINE OIL best viscosity?

[T3R5Z]

I'm still a believer in my own theory as backed up by others and the racing industry. 0 viscosity in 100° weather is too thin. Oil pressure will drop and cause more damage than benefit. The owners manual is based on General ideas and does not factor in any variables. This would be fine in northern cooler temperatures but Not in southern like Florida, Arizona, Nevada where its always hottt. I will go with the dealer recommended no logic other than the corporate written manual.... For now! But going forward the second oil change will be to not kill my motor . "Jet Engine Mechanic"

The 0 is irrelevant that's for cold start. You shouldn't be driving your car hard before it comes up to temp. 0W is going to give you the best possible flow on start-up. The only downside of 0W oil is that if you used it in a car that was not designed for it. Let's say an older engine from the 90s with looser clearances you could loose oil pressure on start-up.

When you modify the engine to produce more heat and then push it hard that's when it could become problematic. But again that's more likely to effect the 2nd number.

 

[speedjuno]

Pure stock, wife’s daily, I hump both when I can.
I quit trying to 2nd guess engine engineers a long time ago. About a million miles, still waiting on an oil related engine failure.

 

[mikecarpage]

Another recent video that explains things

 

[T3R5Z]

Another recent video that explains things

That kind of contradicts EE video. This is the AI answer. I guess if you were having issues on track days you could look at a 0W-30.

I think the safe option is to just do regular oil changes with 0W-20 for normal driving.


This is a common question, and the answer is that neither is "better" in all situations. The "better" oil is always the one specified by your car's manufacturer.
Engines are now designed with extremely precise tolerances, and the oil's viscosity (its thickness and resistance to flow) is a critical component of that design.1


While I cannot show you the full text of paywalled scientific papers from organizations like the SAE (Society of Automotive Engineers), I can summarize their key findings and the established science behind these two oil grades.
Here is a scientific comparison of their properties and functions.

1. Understanding the Numbers: What "XW-XX" Means

The naming convention, established by the SAE, tells you the oil's viscosity at two different temperatures:

• 0W-20:
  • 0W: The "W" stands for "Winter."2 The "0" is the viscosity rating at cold temperatures (tested as low as $-40^\circ\text{C}$). It flows very easily when cold.3
    
    
    
    
  • 20: This is the viscosity rating at the engine's operating temperature ($100^\circ\text{C}$). This is "thinner" than a 30-grade oil.
• 5W-30:
  • 5W: This is the viscosity rating at cold temperatures (tested as low as $-35^\circ\text{C}$). It is "thicker" (more viscous) than 0W oil during a cold start.
  • 30: This is the viscosity at operating temperature ($100^\circ\text{C}$). It is "thicker" than a 20-grade oil.

2. Key Findings from Automotive Engineering Studies

Here is what research and testing have concluded about the performance of each.

Finding 1: Fuel Economy and Emissions

The primary driver for the creation of 0W-20 oil was the global push for better fuel economy and lower emissions (to meet standards like CAFE and those from the EPA).

• Conclusion: 0W-20 is a "low-viscosity" oil.4 It creates less internal friction and "pumping loss" (the energy your engine wastes just moving the oil around).
  
  
  
• Scientific Backing: Dozens of SAE papers and manufacturer studies confirm that using 0W-20 in an engine designed for it provides a 1-2% improvement in fuel economy over 5W-30.

Finding 2: Cold-Start Engine Wear

This is the most critical area and where 0W-20 has a clear scientific advantage.

• Conclusion: Automotive engineering studies have repeatedly shown that up to 80% of all engine wear occurs during the first few seconds of a cold start, before the oil has fully circulated.
• Scientific Backing: Because 0W-20 is significantly thinner at cold temperatures, it flows to critical components like the camshafts, bearings, and valvetrain much faster than 5W-30.5 This establishes a protective oil film seconds sooner, drastically reducing wear at startup.
  
  

Finding 3: Engine Design and Tolerances

This is the most important factor and the reason you cannot simply swap them.

• Conclusion: Modern engines (roughly from 2010 onwards) are built with extremely tight tolerances (the microscopic gaps between moving parts). They are designed for a thin oil like 0W-20.
• Scientific Backing: Using a thicker oil like 5W-30 in an engine designed for 0W-20 can "starve" these tight-tolerance areas of lubrication. More importantly, it can cause Variable Valve Timing (VVT) and cam-phasing systems to malfunction. These systems are hydraulically actuated by the engine oil, and an oil that is too thick will not flow through their small passages correctly, leading to check-engine lights, poor performance, and potential long-term damage.

Finding 4: High-Temperature and High-Load Protection

This is the main area of debate and where 5W-30 appears to have an advantage.

• The Theory: At operating temperature, 5W-30 provides a thicker, more robust hydrodynamic film. In high-load, high-heat situations (like racing, towing, or in a high-performance turbocharged engine), this thicker film can provide a stronger cushion against metal-on-metal wear.
• The Scientific Reality: This is only partially true. While the physical film of 5W-30 is thicker, modern full-synthetic 0W-20 oils are not just "thin oil." They are advanced chemical formulations with powerful anti-wear (AW) additives and friction modifiers (FM). These additives create a chemical bond on the metal surfaces, providing "boundary lubrication" that protects the engine even when the physical oil film is thin.
• Conclusion: For an engine designed for it, 0W-20 provides sufficient, and in some cases superior, wear protection due to its advanced additive packages. Some manufacturers (e.g., Subaru) specify 5W-30 for their turbocharged engines (like the WRX) precisely because they are designed for that higher heat and load, while specifying 0W-20 for their non-turbo engines.

Summary: Which Is "Better"?

Scenario	"Better" Oil	Why (Based on Scientific Principles)
Your Owner's Manual	The one listed	This is the final word. It is the oil the engine was designed and validated with.
Fuel Economy	0W-20	Lower viscosity reduces internal friction ("pumping losses").
Cold Starts (Winter)	0W-20	Flows much faster to critical parts, providing superior wear protection at startup.
Modern Engines (post-2010)	0W-20	Required for tight engine tolerances and correct operation of VVT systems.
Older Engines (pre-2010)	5W-30	Designed with larger clearances that require a thicker oil film to maintain pressure and protection.
High-Performance / Track Use	Check the Manual	Often 5W-30 or even 0W-40, as these engines are built to handle the thicker oil and benefit from its high-heat film strength.

Final Warning: Using 5W-30 in an engine designed for 0W-20 is generally a bad idea and can lead to reduced fuel economy and potential damage to the VVT system. Using 0W-20 in an engine designed for 5W-30 is even worse, as it can lead to insufficient oil pressure and catastrophic wear.
The best "study" to consult is your vehicle's owner's manual.6


Would you like me to find the manufacturer's recommended oil for a specific vehicle model?

 

[toki]

Another recent video that explains things

That video is AI slop that cites to a fake study that doesn't exist. An actual authority on lubrication has already made a video debunking it:

 

[77AkumaNoZetto]

This video is AI slop that cites to a fake study that doesn't exist. An actual authority on lubrication has already made a video debunking it:

Dang beat me to it man!!! LoL

 

[trackratZ]

I'm in warmer SoCal and dealer performed my 980 mi first oil change with M1 5W30. I asked them why not 0W20 and that's what's recommended for this area. Still under warranty so it's their decision and I'm good with it.

 

[kevinbonds]

I'm in warmer SoCal and dealer performed my 980 mi first oil change with M1 5W30. I asked them why not 0W20 and that's what's recommended for this area. Still under warranty so it's their decision and I'm good with it.

Same with me, in south Florida and dealer used 5w30 M1

 

[haohmaru]

Here's a PDF from the USDM 2017 Q60 owners manual that included the temperature range diagram. 0W-30, 0W-40, 5W-30 and 5W-40 were factory recommendations for this motor. Bottom limit for 5W is -13 F.

 

[Gadgetech]

Another, recent video on this topic:

 

[Hernandooo]

Then I wonder why most manufacturers are recalling engines and having their dealers switch from 0w20 to 5w30

Sorry, but cant trust AI on this topic, much less anything vehicle related

 

[chadgigachad]

Then I wonder why most manufacturers are recalling engines and having their dealers switch from 0w20 to 5w30

Sorry, but cant trust AI on this topic, much less anything vehicle related

The engines affected were manufactured incorrectly. The switch to thicker oil is to prolong the life of the engine due to the manufacturing flaw.