Chosen Oil Viscosity...?

I see no reason why SAE 0W-8 wouldn’t be perfectly fine in the Gen 5 Prius. Corolla uses a higher-torque and more powerful version (M20A-FKS: 169 hp @ 6600 rpm, 151 lb⋅ft @ 4400 rpm) of the Gen 5 Prius engine (M20A-FXS: 150 hp @ 6000 rpm, 139 lb⋅ft @ 4400–5200 rpm), and SAE 0W-8 is used instead of SAE 0W-16. Therefore, a thinner oil for the Prius than for the Corolla would be needed. On top of that, the Prius engine would run cooler; so, the oil would be thicker for a given SAE viscosity grade. I see no engineering reason for Toyota’s oil-viscosity recommendation for the Gen 5 Prius. It could be for logistics reasons.

 

Thinner Oil is mainly pushed by the EPA and CAFE to increase fuel economy. Toyota published a paper https://www.sae.org/publications/technical-papers/content/2020-01-1425/. It can formulate 0W-8 with thick enough boundary layer and increase fuel economy by 0.8%.

In terms of fuel economy, 0W-8 is great, but it is only for hybrid without Turbo. Thin viscousity cannot be compensated with coctail of B, Mo, and other additives in high heat high spin turbo shaft and Toyota only use 0W-20 on their Turbo engines for a good reason.

Using 0W-8 won't hurt the engine for newer Toyota that are designed for it, but not for the older one like Prius 4 0w-16 and Prius gen 3 0W-20. On the other hand, using thicker oil such as 0W-20 or 0w-40 are perfectly fine and won't void any warranty in any Toyota that come with recommended 0w-8. In terms of engine protections, 0w-8 is enough, and 0w-20 is also more than enough for Prius gen 5.

Don't sweat it, thicker oil like 0W-40 may protect the engine better if the car is driven under high speed in hot climate zone like middle east or tropical countries. The thickness of oil is exponentially changing over temperature. At 32F, most engine oils are 1000x thicker than at running temperature 100C, and the difference between 0W-40 and 0W-8 at running temparature is minuscle compared to when it just started at below freezing temp. 0.8% over 0W-16 and probably 1% over 0W-40 is enough to make CAFE happy.

I prefer to use whatever Toyota recommend for the car, but if I have thicker oil like 0W16 or 0W20, I will still use them first before buying a new 0W8 as long as it is <5y old from the manufacturing date. Yes, engine oil has selflife in sealed bottle, about 4-5years.

 

It is not just US/CAFE. Many new cars in Europe are also recommending 0W-20. And 0W-16 is coming there, too.

The rule of thumb is to use the lowest viscosity grade that sufficiently protects against wear. This is usually the recommended grade. Use of a thicker oil will lower the performance and the car will feel sluggish, idle more roughly, and will have worse fuel economy.

Thicker oil has its place. It will help with cars that are prone to wear and/or oil consumption, such as some Toyotas with piston-ring problems. It is also recommended for driving very fast or with high loads or towing etc.

Coming back to the recent question, yes, SAE 0W-8 is fine for the Gen 5 Prius HEV/PHEV under normal operation, as it is recommended in the lower-torque version of the same engine in the new Corolla, which would require a slightly thicker oil due to the slightly higher torque/rpm ratio. Prius engines also tend to run cooler because they are off most of the time, which makes a thin oil even more suitable. Last but not least, oil recommendations don’t always follow a strict engineering analysis. For example, Gen 4 Prius HEV/PHEV originally recommended 0W-20 but later model years switched to 0W-16 for the identical car, with the recommendation for the previous model years having been unchanged.