My new Prius

congrats and welcome!
I know so little about engines that I go with mfg's ref's. you'll probably be fine, but don't quote me.
all the best!

 

It's your car and your choice, but I think that's a bad idea.

Let's dive into exactly what you think moving to a higher viscosity oil will do. You're thinking that the higher viscosity oil will reduce wear on internal components, thus extending engine life. But that argument only makes sense if the primary early failure mode of the engine in question is bearing wear. If the bearings are the weakest link in the engine. But that's not the case in Toyota eco-4s(any 4-cylinder, non-turbo engine meant for economy: 2ZR, 2AR, M20A, A25A, etc.). When these engines catastrophically fail, it's not from oil viscosity, it's from oil starvation.

Let's go through the typical catastrophic failure mode of a 2ZR in a gen2/3/4 Prius, and let's think of it in terms of a set of dominos. The first thing that happens is that the piston rings start to clog up from oil sludge. After the sludge builds up enough and hardens, the low tension rings(low tension for fuel economy/emissions) lack the strength to fight the sludge and can no longer properly seal the combustion chamber.

Now the engine starts to experience blowby and oil consumption. Fuel is more likely to get past the rings and start mixing with the oil, and oil is getting past the rings and being burned off. When the rings are locked off center, you'll also see extra wear in the cylinder walls, allowing even more fuel and oil to get past the rings.

At this point, you see a split of scenarios depending on the owners. Good owners keep their cars going by constantly checking the oil level and topping it off to counter for the consumption. Eventually there's enough gap from the rings that compression gets low enough for misfires and the engine needs to be replaced. The other scenario is owners that don't check their oil enough. In this scenario, the oil level drops low enough that there's an oil starvation event(or several) and the bearings seize, causing a thrown rod or other catastrophic damage.

But here's the thing, if you pull apart a Toyota eco-4 that needs to be replaced due to misfires from low compression from cylinder scoring, you'll usually find the bearings are still in great shape. 150k-200k engines that have a history of terrible 10k-20k oil changes and excessive oil consumption will still have perfectly good main and rod bearings. They're not a weak link in the engine. The only time the bearings fail is when there's no oil. (of course there are the rare exceptions to this rule, but we're talking overall trends here)


Going back to the domino thing, what we want to do is stop the dominos falling as early as possible; preferably keeping the very first domino from falling.

What will higher viscosity oil do? Well, it might prevent bearing wear, but the bearings are already one of the strengths of these engines. We don't need to address the strengths, we need to address the weaknesses. Higher viscosity oil might also lower oil consumption a bit once the rings are locked up and cylinder scoring has started, but that's over halfway down the domino run and it's only treating a symptom, not a cause.

What we really need to do is stop that very first domino: the sludge in the ring grooves.

How do we do that? Two ways: First, religious 6m/5k oil changes. We want to keep the oil clean and full of detergents. We want to slow the rate of sludge buildup to a crawl. Second, we take advantage of the newest oils that can actually remove deposits rather than just slowing them down. Valvoline Restore & Protect(VRP) has been out for a few years now and is showing good results. They also just came out with it in 0W-16. Also, Mobil 1 has just come out with their Advanced Clean oil to compete with VRP, but the lowest viscosity of it so far is 0W-20.

Rather than going crazy with something like 5W-30, I'd go with either Toyota 0W-8 or VRP 0W-16 and do 6m/5k changes. Keep those rings clean and keep the first domino from falling. That's how you're going to get the longest life out of your engine. (My 2023 calls for 0W-16. I used Toyota 0W-16 until my last oil change this spring when VRP 0W-16 became available.)



A side note is that modern Toyota eco-4s like the M20A in the gen5 all use variable displacement oil pumps and have at least some solenoids that function by oil pressure. Those parts are programmed for a specific oil viscosity. If you change the viscosity too much, you might introduce new problems. In other words, beware the law of unintended consequences. Even though the same car in a different market might call for a different viscosity, you don't know for certain if those cars come with slightly different engine computer programming to account for it.


Anyway, that's my opinion based on what professional, long-time Toyota technicians like The Car Care Nut say. They've seen and replaced thousands or tens of thousands of these eco-4s over the last 20 years, and they say stick to the manufacturer recommended viscosities and just change it more frequently. Clean oil is better than thicker oil.

 

You stick with what Toyota says to put in the crankcase. Since you are buying a new Prius. You do not want to do anything that will void the warranty. Do not attempt to do anything until your Prius is out of factory warranty. Let the dealership do all the servicing.

 

Not sure what the Brazil market had to do with your thinking but it got me to wonder because I had already known Toyota doesn't even sell the Prius in Brazil.

I don't know, can one reason be that the owner's manual states the M20A-FXS engine is NOT to be used with any flex-fuels or gasoline that could contain more than 15% ethanol and Brazil seems to have a fundamentally different infrastructure than the USA where flex-fuels seem to be the norm with blends in ranges between 27-95% ethanol?

And unlike the Prius which are all made in Japan, I seem to have found that Brazil manufactures a variant of that engine they call the M20A-FXB (Flex-Fuel Brazil) that's made to work with their fuel infrastructure.

So, maybe there's a relationship between the fuel in Brazil and the engines made for that market and engine oils they use that is more protective of all that ethanol?