5th gen prius 0w8 vs 0w16 discrepancies

I know you really want to win your argument about it being okay to put 0W-8 in the car.

And it's probably true that it is okay - that was widely surmised when the Prius first came out, given that other models with M20A-FXS were already specced for 0W-8, and it's even more likely now that they've changed to 0W-8 for the Prius since July 2025, with no obvious design changes, aside from the oil level sensor removal, which could be coincidental.

But that's no reason to keep making stuff up. You've got a reasonable claim based on the engine type, but it doesn't help your credibility to invent things about the Prius production.

The manuals and labelling for Japanese and European cars all said 0W-16. There's no evidence of 0W-8 ever being part of Prius production until July 2025 (aligned with the US 2026 model year).

I know about the European labels myself, having one, and I just spent some time browsing Japanese YouTube for a shot, here:




And sure, maybe the car could have been filled at 0W-8 at the plant, despite all documentation and on-car labelling saying 0W-16, but there's no evidence for that. It could also have been filled with 5W-30, right?

The pre-2025 English and Japanese manuals do specifically say, "SAE 0W-16 is filled into your Toyota vehicle at manufacturing" (and "0W-16 は、新車時に工場充填されている推奨オイルであり"), so you're making a claim that the documentation is actually false, with no evidence.

Personally, if I had to guess, I'd say that they were being cautious about the use of 0W-8 in PHEVs - they may not have had enough experience with it to see if it coped with the sort of short cycles it might get, so they wanted to stick with the 0W-16 they had more experience with, at least for the PHEV. And then to simplify the Prius production, both HEV and PHEV got the 0W-16, rather than using different oils for the two variants on the line. And then few years later they'd managed to do enough testing with 0W-8 to be happy with it.

 

Above pic was from here, btw. A two-and-a-half hour review video

 

Regarding protection, chances are that you get a marginally better wear protection when you go to higher viscosities, as it increases the oil-film thickness. In particular, base-oil viscosity determines the protection in the valvetrain and parts of the cylinder rings. So, technically, you could see better and better wear protection when you go from 0W-8 to 0W-16 to 0W-20 to 5W-30.

However, chances are that you won’t see much difference unless you are driving very fast. In fact, Toyota recommends using higher viscosities in the owner’s manual if you drive/accelerate fast.

One nice thing about using the lowest recommended viscosity is that the oil is thinner when it is colder, which improves cold-engine fuel economy significantly. I don’t like pushing the engine to its limits when the oil is still too thick, as it could lead to air gaps (foaming/aeration) in the oil when the rpm is too high and the oil is too thick to flow.

Regarding performance upgrades, ILSAC/API upgrade their specs roughly every five years. It is currently API SQ/ILSAC GF-7A/B, which came out in March 2025. The previous update was in 2020. I don’t know if TGMO 0W-16 has been updated to API SQ/ILSAC GF-7B yet.

0W-8 is still not part of ILSAC/API but JASO. The new SQ/GF-7 additive package is actually better than the JASO GLV-1 additive package, which is because JASO GLV-1 was introduced earlier. However, JASO GLV-2 was introduced recently, and it should be comparable in protection to API SQ/ILSAC GF-7. I see that the 2026 Prius HEV/PHEV still specs the inferior JASO GLV-1; so, the GLV-2 update has nothing to do with the mid-cycle oil-recommendation change. I don’t know if the American TGMO 0W-8, which is made by ExxonMobil, has been updated to JASO GLV-2 yet.

 

Actually, after reading the link I gave for the new JASO GLV-2 spec, it is a more watered-down 0W-8 that prioritizes fuel economy more at the cost of wear protection and oil consumption. What they did was that they further lowered the base-oil viscosity, which further compromises wear protection as I explained in my previous post, and added more viscosity-index improver (VII) polymers to have a more “flat” viscosity behavior so that the viscosity is less dependent on the temperature (meaning a higher viscosity index VII). While this sounds good to the ear, it compromises wear protection. It also compromises oil consumption as the base oil is further thinner now. To get around the volatility specs, they introduced a new volatility test instead of the well-established Noack volatility, which reduces the test temperature from 250 ℃ to 150 ℃. I am guessing the Noack volatility of JASO GLV-2 oils could be as high as 20%.

So, use the new JASO GLV-2 SAE 0W-8 oils at your own risk. LOL

 

Missed the above due to Christmas.

Yes, the M20A engine was designed for 0W-8 oil, we all know that.

The OP has filled the thread with paragraphs of true statements about the M20A engine, and false statements or hallucinations about the Prius.

Most of the technical claims in the last two posts are true - the stuff about the M20A.

This statement is misleading:

The recommendation for the Prius does not change by region. 0W-16 initially globally, 0W-8 from July 2025 globally.

It's possible that the recommendation for other cars changed by region, in the 2021-2022 period. I think this was the case with some Corollas? I believe there were supply chain issues meaning Toyota US were advising 0W-16 while other countries were filling the same cars with 0W-8.

But that's not the case for the Prius - it used 0W-16 globally.

I've seen no evidence that Toyota's claims to have filled the Prius with 0W-16 in initial production are wrong. The engine was engineered for it, yes, but engines don't come pre-filled with oil - they're fitted to a specific car and filled on its production line.

On the evidence I can see, it's more plausible that the Prius line filled M20A engines with 0W-16, for some reason, than that they let tens of thousands of cars go down the line with mismatching on-car plaques and incorrect manuals. That's not the Japanese level of attention to detail!

It would be very interesting to find out why they decided the Prius should use 0W-16, despite other cars with the M20A using 0W-8. (My guess above was that it was a PHEV thing, but that's just a guess).

It would be even more interesting to find out that the OP is right and the Prius was filled with 0W-8 and all the documentation was wrong, and the July 2025 change was just a quiet manual correction. This is possible, if unlikely. (If the OP actually knows this from a source, it would have saved time to post it. Extraordinary claims do require evidence.)

And it would be interesting to see if there was a retroactive notification to use 0W-8 on 2023-2025 Priuses in future. That seems perfectly possible.

Now, I've gone through the thread one more time, looking for any actual evidence, and the only thing I can find that might be something is the reference to "T-TT-0692-23. This is the internal Toyota memo that tells technicians 0W-8 is the "new" standard for these engines."

I can't find this document publicly - maybe someone can show it? My current guess is that it's about other cars with the M20A engine, not the Prius. I think it must be something issued in 2023 saying that Corolla Hybrids and other M20A cars that were filled with 0W-8 at the factory and recommended for 0W-8 outside the US should now be filled with 0W-8 inside the US.

In which case not relevant to the Prius's global 0W-16 fill and recommendation from 2023-2025, which was dictated by Japan, and was hence nothing to do with Toyota US's already-obsolete 0W-8 supply reticence.

All of this is a bit of pointless argument - I'm sure it's absolutely fine to use either 0W-8 or 0W-16 in a 5G Prius, and I bet any difference is barely perceptible. I would agree totally with the statement 'By using 0W-16, you are essentially using "yesterday's tech" in "tomorrow's engine."'

But I'm not going to let statements of fact about something concrete like Toyota's production and recommendations go past if they're false (or at least extraordinary and unevidenced).

 

Actually, I've just noticed the ambiguous wording on this quote:

It's similar to the previous misleading quote, but I didn't parse this one carefully enough.

I thought OP was restating the claim about the Prius being filled, but I think it's now a new true general statement about "the engine" - meaning M20A in general rather than the engine in the Prius. Certainly many M20A engines were filled with 0W-8 in Japan.

Reading very carefully, I think maybe OP has conceded that the Prius wasn't filled with 0W-8, but tried to hide it in noise about the M20A generally.

 

This is actually simply false. The engines come prefilled with oil from the engine factory. They are not filled with oil during final assembly.

How do I know? I wasn't sure what oil they had put in the factory when my new 2009 Barcelona Red Metallic Corolla XLE was manufactured in early 2008. Toyota had recently switched to 0W-20, but there was an ambiguity. So, I called the GM/Toyota NUMMI assembly plant, which is now the Tesla factory. They were very friendly, verified that it was 0W-20, and told me the engines came prefilled with oil from the engine factory.

 

Interesting thread.

I have no insights to OEM original oil viscosity fill or the manual and "under hood" sticker viscosity statements.

I've read 0-8 is better for cold weather and fuel economy.
0-16 may be better for hot weather or prolonged highway speed or a heavy loaded/ cargo Prius.
Which is preferred, I do not know

Again, interesting thread.