The difference is how thick each oil is when cold and when hot. Under the SAE J300 viscosity standard, 5W-20 and 5W-30 meet the same cold-temperature limit, so both flow faster at startup than 10W-30. At operating temperature, 5W-30 and 10W-30 share the same viscosity grade, while 5W-20 is thinner for better fuel economy.
Let’s look at 5W-20 vs 5W-30 first. Then we’ll look at 5W-30 vs 10W-30.
The primary difference between 5W-20 vs 5W-30 is their viscosity once your engine has reached operating temperature.
Notice the different numbers in the viscosity ratings: “5W” and either “20” or “30”. The first number (5W) refers to the oil’s viscosity when cold, measured at -30°C (-22°F).
Video transcript: Choosing the Right Motor Oil Viscosity
Featuring John Gardner, host of Motorhead Garage, and Leonard “Len” Groom, AMSOIL Director of Product Marketing (author profile).
John Gardner: Hey. Just checking out this refractometer reading. You guys wanna take a look?
Bam. What are you looking at? You’re actually looking at the weight or the specific gravity of fluids. You can see battery, glycol, different coolant, stuff like that. Super important and that’s important when it comes to oils because we’re talking about viscosity, Len.
Len Groom: Viscosity is an important piece of oil technology.
John Gardner: I’ll tell you what, let’s have a little bit of a race, man. What do you got right there?
Len Groom: Well, I’ve got an SAE sixty racing oil.
John Gardner: Alright. And, I’ll use the actual zero w sixteen.
Len Groom: I feel like I feel like I’m gonna lose, but we’ll explain that in a minute.
John Gardner: Yeah. You might lose, but let’s take a look at it. So, this is the thick one, and this is the thin one. According to that viscosity rating, let’s see if that’s actually true. What I’m gonna do is I’m gonna drop these balls in and we’re gonna watch them go down. So, in three two one There they go.
Man, look at the difference. That one went down in no time. Len, you lost. Why is that?
Len Groom: Well, I lost because that sixty weight is a is a thicker oil.
John Gardner: It is a thicker oil and our chart actually showed you that perfectly. The high viscosity, the slower the flow. Lower viscosity, faster the flow. Even more important, starts to mess with temperature. This is huge because you’ve got the twenty w fifty on the top and the zero w thirty on the bottom end, but there’s a huge range difference in between there. Why is that? What’s temperature have to do with it?
Len Groom: Well, as you can see, you know, for starters, it should be noted that engine oil, that second number, the one that indicates the viscosity, is taken at two hundred and twelve degrees f. That’s an industry standard. It’s also generally the operating temperature of the oil in your car. But as you start to look at those lower numbers, your twenty w, your fifteen w’s, your tens, that indicates how the oil moves at low temperature. The higher that number, the slower it’s gonna move. So a twenty w fifty is not going to move well at low temperature.
Reverse that, a zero w thirty say is gonna move very well at low temperatures. Why is that important? Because of the technology that we’re dealing with on some of these new vehicles.
John Gardner: And that’s even room temperature. I mean, you can hear it here. That one kinda sloshes around.
Not as much. Pretty thick. Yeah. Makes a big difference. Vehicles are sophisticated today and your oil has to be.
You know, you got systems like displacement on demand, variable valve timing. You got timing chains that are driven by oil. Use the manufacturer specified viscosity rating. If you’re not sure which oil to use, no worries.
Just go to AMSOIL.com. The viscosity and quantity, they’ll tell you everything, and they’ll give you the best oil for your vehicle.
5W-20 & 5W-30 vs 10W-30: The 5W Oils Flow Faster In Cold Weather
It helps to think of the “W” as standing for “winter.” Lower numbers mean the oil will remain more fluid and easier to circulate throughout your engine for improved start-up protection.
That being the case, when comparing 5W-20 or 5W-30 vs 10W-30, the 5W oils will flow faster in cold weather, as indicated by their lower 5W rating.
This means the oil will flow more readily to lubricate the cam lobes, piston rings, cylinders, bearings and other vital components at startup. The sooner the oil reaches these components, the sooner it can protect against wear.
Oil that’s too thick can cause precious seconds to pass when little-to-no oil is available to lubricate parts, resulting in wear.
It’s especially important in sub-zero weather when oil naturally thickens. Synthetic oils remain more fluid in cold weather for better protection than their conventional counterparts. Check out this post for more about the differences between synthetic and conventional oil.
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AMSOIL Staff articles draw on the combined expertise of AMSOIL engineers, chemists, Technical Services specialists, technical writers and product marketers. Content is developed from AMSOIL data sheets, laboratory and field testing, oil-analysis results and more than 50 years of synthetic-lubricant development. Topics span automotive, diesel, powersports, marine, small-engine, agricultural and industrial equipment, covering lubrication fundamentals, maintenance practices, product selection and real-world customer experience.
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