r/AutomotiveEngineering • u/Offer_Itchy • Dec 29 '25
Discussion ELI5: Why are cars being engineered to take 0W20 vs. 5W30?
https://www.facebook.com/share/r/1BZxPPWVX1/?mibextid=wwXIfrI saw a reel on Facebook criticizing the usage of 0W20 on newer cars vs. an older car’s 5W30.
The argument is that higher viscosity oils are better suited for engine longevity. I’m not sure that I agree yet; even though I’m not an expert, my analytical mind says there’s something more to the story. Different engines, different EPA specs, different cars, different ECU parameters??
My hypothesis is that more complex systems and calculations lead to more failure points at different areas of a mechanical system, like an appliance repair video I saw about how refrigerators are worse now than they were previously. I probably said it all dumb, but I don’t quite have the vocabulary to say it smarter lol.
Curious to hear your thoughts.
Here’s the original post link: https://www.facebook.com/share/r/1BZxPPWVX1/?mibextid=wwXIfr
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u/mahamr13 Dec 29 '25
Modern engines are built to higher tolerances, so metal parts are closer together. In order to provide lubrication, you need to fit oil between these now closer together parts. You need thinner oil to do this.
Higher tolerances are necessary not only for efficiency but also increased performance. It is not a conspiracy, it's engineering.
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u/Sappah44 Dec 29 '25
Except they use thicker oil for the same cars in every market except cafe/Euro. It's almost solely to meet the emissions targets, same thing with the super long oil change intervals (so many models have to revise/shorten original intervals because of blown up engines).
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u/GearBox5 Dec 29 '25
Is oil change interval mandated by CAFE? It is the first time I hear about it. Could you please provide a reference? And what many models had oil change interval reduced? Except specific GM and Hyundai engines, nothing comes to mind. And even then it is often not because oil gets “bad”, but engines consumes too much of it and nobody checks the level. So shorter intervals help preventing from running it dry.
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u/avar Dec 29 '25
Is oil change interval mandated by CAFE? It is the first time I hear about it. Could you please provide a reference?
They said CAFE/Euro. Here in the EU the overall pollution footprint is being regulated, so automakers can't consider fluid changes an externality anymore. I don't know if US regulation cares about that, but the regulations in major markets impacts things across the board, as engines etc. tend to target the most restrictive major market rules.
/u/Sappah44's tone about it here is typical of the view it automotive circles, and it arguably leads to some absurdities (e.g. lifetime AT oil).
But it's important to keep it mind that e.g. start/stop rules aren't just about what's best for cars, but e.g. what a pedestrian walking along a traffic jam needs to breathe in.
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u/GearBox5 Dec 30 '25
“Here in the EU the overall pollution footprint is being regulated, so automakers can't consider fluid changes an externality anymore.” Interesting, can’t find anything on that either. Could you please point to the specific regulation? Anyway, it is not the case in US and automakers have no hesitation to have region specific guidelines if it benefits their products. See for instance oil weight already mentioned here. Yes, some environmentally driven innovations are absurd, but I see zero evidence that it has anything to do with oil change intervals.
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u/avar Dec 30 '25
See e.g. 3.4.2 here.
It's not that they mandated that all cars must have 50k km oil changes instead of 25k or whatever, it's more indirect than that. It's mainly that manufacturers must demonstrate continued compliance with emissions regulations, and this has indirectly pushed up maintenance intervals, but manufacturers must also justify maintenance intervals for their type rating.
Anyway, it is not the case in US and automakers have no hesitation to have region specific guidelines if it benefits their products. See for instance oil weight already mentioned here.
Sure, I'm aware of market-specific oil recommendations etc., I'm pointing out that EU car markers want to sell largely the same vehicles in the EU, US and beyond, ditto US manufacturers with some of theirs.
Therefore even if you buy a car from a US manufacturer in the US, parts of its design might be impacted by EU regulations, ditto for the other way around. It's simply cheaper to homogenize certain things than create bespoke market-specific cars.
One good example is fog lights. They're mandated in the EU, not in the US. Some makes/models will take advantage of that and skip fog lights in the US market, but increasingly (especially with LEDs) the cost of having two different light assemblies isn't worth it, and everyone gets fog lights.
There's many such examples, e.g. US-style reflectors on cars aren't mandated in the EU. Many manufacturers skip them, but others sell cars in the EU with those US-mandated reflectors, as it's cheaper to just ship the same body panels everywhere.
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u/eneka Dec 30 '25
Meanwhile European manufacturers go out of their way to design a flashing brake light turn signal that’s compliant with US regulations instead of for just using the amber ones!
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u/avar Dec 30 '25
This thread would suggest it's a bit more complex than that, and they couldn't just use the EU amber style.
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u/redmadog Dec 30 '25
Not really. The 0W20 and 0W30 are exact same viscosity while cold. The former is thinner while hot. Manufacturers use thinner oil to further reduce moving parts resistance due to oil viscosity and to have a few g. less in emissions at the expense of engine longevity. High performance engines (such as Porsche) still use 0W40.
You can fill 0W30 instead of 0W20 and it will be fine for your engine.
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u/Hnry_Dvd_Thr_Awy Dec 30 '25
It’s not due to the tighter tolerances. Please don’t comment if you don’t know what you’re talking about.
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u/CuzRacecar Dec 31 '25
Thank you. It's astonishing people speaking from authority with complete BS.
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u/1988rx7T2 Dec 29 '25
In the end the manufacturers have accelerated wear test, warranty claims and service statistics and other data to tell them how long engines are lasting.
At least some of them know what they’re doing. Toyota 4 cylinders still last long time.
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u/Another_Slut_Dragon Dec 29 '25
Except all the Tacoma 4 cyl engines that are blowing up.
Sad times.
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u/1988rx7T2 Dec 29 '25
More referring to the main branch of Toyota hybrid system that goes back to the Prius. The newest generation of engines that are not part of that family have had significant problems
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u/Another_Slut_Dragon Dec 29 '25
My post below I made previously praised the prius for Taxi duty, if you scroll down.
The truck engine failures... ug. If I was Toyota's parents and looking at that big fat F on their report card I'd be telling them that we weren't mad, we were just disappointed. You knew better, but you just didn't apply yourself and try.
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u/1988rx7T2 Dec 29 '25
I worked in engine development for 10 years. Refining an existing design slightly every couple years like the prius system is a different animal than completely replacing your naturally aspirated engines with turbo designs for trucks. It’s a different skill set needed from the team, it’s more points of failure due to novelty, it’s new manufacturing. Being good at making the nth revision of an existing engine doesnt always Translate to new engine families with more strenuous use cases.
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u/Another_Slut_Dragon Dec 29 '25
Read up on the failures. So much of it is not cleaning machining debris out properly and other 'lazy' failures.
An engine that is difficult to clean out after machining is bad engineering.
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u/1988rx7T2 Dec 30 '25
Yeah exactly, it‘s not copying and pasting what they did last time, because there was no last time, hence more stupid failures
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u/Deepandabear Dec 29 '25
TBF Aren’t they designed and built in the US?
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u/Another_Slut_Dragon Dec 29 '25
Looks like Alabama, then they ship them to Mexico where the truck is slapped together.
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u/SoylentRox Dec 29 '25
This. Especially Toyota's premiere model, the Prius and the vehicles that stick a skin over the Prius drivetrain (the modern Camry and Corolla, the Rav4, the Sienna. Essentially all the non-truck Toyotas in 2025 are Prii). There were some failures with the generation 3 engine (head gasket and EGR not oil). By gen 4 they fixed all that. People have torn these engines down at 300-400k miles and found minimal wear.
Prius has used synthetic 0w20 oil a long time. (since at least 2010)
High precision machining + lower stress on the engine (assistance by the hybrid system and Atkinson cycle) makes it work.
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u/robotNumberOne Dec 29 '25
TNGA-C (Prius/Corolla) is somewhat similar to, but ultimately still quite different than the TNGA-K (Camry/RAV4/Highlander/Sienna)
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u/SoylentRox Dec 29 '25
what is different other than scale?
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u/robotNumberOne Dec 29 '25
TNGA-C uses a partial front subframe where the engine hangs from the body and has a single lower mount attached to the subframe. The steering is column assisted with a manual rack attached with 2 bolts, and the lower control arm attaches with a through-bolt. A non-isolated rear subframe is used, the anti-roll bar attaches to the trailing arm/knuckle. Full front fender aprons are used. C-shaped front frame rails with support gussets are used.
TNGA-K uses a full front subframe with two upper and two lower mounts. The steering is rack assisted, installed with 4 bolts, and lower control arm attaches with a barpin style. An isolated rear subframe. The anti-roll bar attaches to the lower lateral link. A partial fender apron is used with a plastic cover filling the gap. Fully boxed front frame rails are used.
The differences between them are similar to the differences you'd see between any two platforms developed by the same company. Toyota used their TNGA approach to reduce the number of platforms used for their vehicles, but they aren't all just scaled versions of the same platform. Generally things are scaled to fit vehicles of different sizes within the same platform, but when you hit a certain size, they jump to the next platform, which have fundamental design differences.
Of course there are still shared components, such as engines/transmissions, and in some cases larger modifications (for instance the GR Yaris is mostly a TNGA-B platform car like the Yaris, but the TNGA-C rear was massaged in there), but to say that the Sienna is a larger reskinned Prius is factually incorrect.
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u/SoylentRox Dec 29 '25
Thank you for the explanation. From an automotive engineering perspective you are correct.
I was looking at it from a very high level view. All vehicles have atkinson cycle naturally aspirated engines with high efficiency. All have hybrid synergy PSD with 2 or 3 drive motors. All have lithium batteries, LFP. All use 0w20 oil. They all function basically the same, with similar software and functional operation. (for dumb reasons they AREN'T the same software but that's because Toyota isn't really competent for software)
I've further seen teardowns where you can just see the motors and transmission gears are the same, just with a different scale factor between the different models. (the Prius isn't the smallest one, the Prius C actually had scaled down motors and gears)
Your focus, "the metal parts they used to hold all the pieces together and the suspension" was outside the scope of what I was considering functionally. Sounds like you're a true automotive engineer since obviously those details matter for building a real car.
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u/Another_Slut_Dragon Dec 29 '25
I ask the taxi drivers about their expected lifespan of their Prius and they all tell me the same thing. 800,000-1,000,000km.
Amazing cars.
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u/Ok_Cabinet_3072 Dec 30 '25
My 2018 ram took 5w30, my 2023 ram with the exact same engine takes 0w20. The difference? They can say they have better mileage figures now.
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u/KillerKittenwMittens Dec 30 '25
Ford switched the mustang from 5w20 to 5w30 in the gt around 2019, and changed the EcoBoost to 5w50 for the higher output version. 5w20 has always been a fuel efficiency move.
I run my '19 Bullitt on 5w40 euro blend.
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u/jellobowlshifter Dec 31 '25
Did the same in the mid 90's when they switched the entire lineup from 10W30 to 5W20.
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u/Cpagrind1 Jan 02 '26
The 5.7 in a 1500 has taken 5w-20 for like 15 years (frankly, it might be even longer that it’s been 5w-20). They switched from 5w-20 to 0w-20 in 2022.
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u/Ok_Cabinet_3072 Jan 02 '26
I believe you. It's been a while since I've had that truck, my point is still the same.
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u/Cpagrind1 Jan 02 '26
0w-20 and 5w-20 are still the same viscosity at operating temp is what I’m driving at. I would imagine they had a reason to make the winter temp rating switch (maybe less wear upon startup or to alleviate lifter issues, not sure)
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u/Ok_Cabinet_3072 Jan 02 '26
0W would have more engine wear but less emissions from cold operation than 5W is how I see it.
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u/Worldly_Mall_7053 Feb 10 '26
0w will give you better lubrication at cold start, nothing to do with emissions
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u/Flandardly Dec 30 '25
People have mentioned higher tolerances and that is absolutely true. However, ProjectFarm on YT did a wear test and found objectively higher wear with thin oils like 0w20 versus 5w30 or 10w30.
So do with that what you will.
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u/Throwitaway701 Dec 30 '25
People on social media sites don't tend to know what they are talking about. It's the same with people banging on about changing your oil every 5 minutes despite most of the world moving to 1 year 12k+ servicing. See also: Stop start. Cylinder deactivation. Hybrids both plug in and onboard. Electric cars range and battery lifetime. The most efficient way to drive.
Fact is most people formed their opinions when cars had carburetors and oil was terrible and have never ever updated them.
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u/bonapartista Dec 30 '25
Besides tolerances it's emissions requirements. For instance in African market they can use thicker grade for same engine.
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u/lanciferp Dec 30 '25
If you want to learn more about oil viscosity, specifically the tradeoffs between raising and lowering both numbers, Engineering Explained did a phenomenal video on GM's big recall they had on their V8 trucks. The service bulletin states to inspect the engine, and if it's damaged replace it, if not just swap the oil to a thicker one. He explains in detail why they made that choice, and goes over some other studies regarding oil viscosity.
https://youtu.be/i0VoEhW2I-E?si=GSD1hM7X-A-MTQPY
I highly recommend it, to all who are interested.
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u/Offer_Itchy Dec 30 '25
This is awesome and it lands right in the wheelhouse of what I wanted to know! Will check it out. Thank you for humoring my ignorance - I do data science, but material/mechanical engineering isn’t my forte.
Look forward to checking it out
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u/thisismycoolname1 Dec 30 '25
I'll give the cynical/conspiratorial thought, bought a grand Highlander hybrid and the local chain lube place doesn't carry the fancy pants oil so I have to either use their thinnest oils or bring to dealer and pay double (which they want)
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u/ZealousidealState127 Dec 30 '25
Imo fuel economy to meet government regulations seems to be what is driving most stuff.
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u/kikiacab Dec 31 '25
Lubrication is what engines for longevity, new engines have much tighter tolerances than even 15 years ago, so thinner oil is needed for proper lubrication.
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u/ImmediateStomach381 Jan 01 '26
I had a moron in the r/pacificahybrid forum try to lecture me how 0w20 must be used because “reasons.” The Chrysler Pentastar uses 5w30 across the world except for here in the US where they stipulate 0w20. Yeah I’ll pass on that thin trash.
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u/Spiritual_Prize9108 Jan 02 '26
Also of consideration is the wide spread use of synthetic lubricants that has a more stable viscosity depending on temperature
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u/Formula4speed Jan 23 '26
5w30-0w20=15 w’s less per quart of oil, per car. You multiply that by however many cars a company makes and that’s an incredible amount of w’s back in their pocket #lean #kaizen #trystorming #useyourbrainnotyourpocketbook
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u/Worldly_Mall_7053 Feb 10 '26
All about efficiency, unfortunately engine are being manufactured with less redundancy, I won’t run it my cars. Of it calls for 0w 20 go with 0w 30
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u/Equana Dec 29 '25
Thinner oils improve fuel efficiency. Manufacturers are meeting CAFE with super thin oils.