The chart I attached to this post is off a Mobil 1 Extended Performance oil specification page. Im attaching this just so it's easier for people to understand what I'm about to say.
Look at the 2 cSt viscosity rows. 0w-20 at 40 degree Celsius is at 44.9, and at 100 degrees Celsius it's at 8.6.
Now let's look at 10w30. 10w30 at 40 degrees Celsius is at 63.7, and at 100 degrees Celsius it's at 10.2.
The there isn't too huge of a difference between 0w20 and 10w30 when both oils are at 100 degrees Celsius. (100 degrees Celsius is 212 degrees Fahrenheit).
There is a much bigger difference with the different W ratings than the operating temp ratings.
The lower the cSt number your oil is at when it's 40 degree Celsius, the better. For example, if your engine takes 10w30, you can just use 5w30 or 0w30 no issues at all, because 10w30 at operating temp is only 10.2. You don't need 63.7 cSt when the oil is at ambient temp. You can safely go down to 0w.
Why do people think they need 10w30 oil with 60+ cSt rating when the engine is cold, but then the same engine needs a 10 cSt rating when the engine is upto operating temp?
Many car manufacturers are recommending 0w oils at all ambient temps nowadays. And no they are not trying to "scam" you or make your engine wear out faster. 0w is good even in hot summer days, and it's also better in colder climates.
There is no reason to use an oil with 60+ cSt when your engine only needs 8-10 cSt in operating temp. You theoretically need 10cSt when the oil is cold and hot. No need for 10w.
Now if you live in places with an ambient temp of 100C or greater (which 99% of the world does not live in places with an ambient temp greater than 40C), then the 0w can thin out too much, so that's why manufacturers generally recommend thicker W rated oils in hotter climates like the middle east.
Ideally, if we had an oil that can behave like 10cSt when the oil is cold (at an ambient temp of 40C) and when it's hot (at an operating temp of 100C) it would literally be the best oil money can buy. But that's just not possible because it's just isn't possible to do that. Maybe one day in the future that will be possible. Oil gets thinner with heat. It's just how it is and we can't change that.
So the "W" matters, but for most of us, 0w and 5w is more than good enough, and 10w is just unnecessary, and it will just cost you more in fuel. And yes, oil viscosities do change your mpg.
Also, I've never heard of anyone that used 0w oil in 100F summer days and experienced any engine problems. Millions and millions of cars worldwide use 0w oils nowadays, and many of them are lasting 300k miles or more no problem at all, as long as they change the oil at a drain interval that makes sense. Following proper oil drain intervals are way way more important than thinking about "should I use thin oil or thick oil."
I use 0w20 in my 3.5 Ecoboost. Yes you heard that right. It's not a typo. I use 0w20 in my 3.5 Ecoboost engine, which normally is supposed to run 5w30 according to Ford. I switched from using 5w20, 5w30 and 10w30 to 0w20. I've been using 0w20 for almost 20k miles now, because I do believe that 0w oils are objectively better in cold starts (in all ambient temps). I believe that there is no need to use anything with a thicker W rating, because of the objective reasoning I stared above. My car has 100k miles, and my cam phasers are still the original, timing chain is original, and there is no rattle or any of the common problems many Ecoboost users have had over the years. My car runs perfect, and I plan to use 0w20. And I live in South Carolina. It's really hot here in the summer (easily 100F in the summer), and 0w has been great in my engine.
Just my two cents.
And now I just chill out with a nice big bag of 🍿 and just wait until mobs of die hard bitogers that literally anxiously spend 12 hours a day on bitog "accurately researching" about thick vs thin to literally annihilate and cuss me out through the screen.
🍿
