I work at a huge ford dealership, and I know we will do anything we possibly can to void the warranty, and I mean anything. It’s the truth. I’m not a technician or a “lube tech”, I don’t do any hands on repairs, but I know the truth because I work for the service department and I know everybody that works at service. I do not put out my real name or ever give out my location on the internet, or even on social media because some dealerships do spy on their employees’ social media accounts. I work at a dealership that does this. I’ve had one coworker get fired for “talking s**t”on the internet, and for telling people the “secrets”. It’s sad, but very real.
Enough ranting, I’ll get to the point.
I highly doubt any dealership would go through the trouble of finding if the right oil was used, but the best thing to do is just stick to the owners manual and use the “recommended oil” until the warranty is over. It may not be the best thing to do for your car’s reliability or longevity, but it is the best thing to do if you do want to actually take advantage of getting the repairs done that are covered under the warranty, if you actually do need it.
To be honest, I probably would use 0w20 or even 0w30 so I can sleep easier at night, I’m not gonna lie, but 0w8 or 0w16 should be more than adequate for a daily driven Corolla.
A good thing to do is just get all the services done at the dealership until the warranty is over. If anything goes wrong with the engine, there’s no if and or buts, the dealership will have to repair whatever that needs to be repaired (that’s covered under the warranty) no questions asked. There are certain cases where crazy questions will be asked, but that’s a whole different topic.
0w8 will work just fine for a daily driven Corolla. I really don’t think Toyota would recommend 0w8 knowing the engines will be toast right after the warranty expires, or won’t last at least 300k miles. That’s just not Toyota. Any oil will work, but I personally don’t really see a need to use a different viscosity. I’ve heard in Japan, Toyota has been using 0w8 for a long time, and they seem to be doing just fine. In the US, we do lots of highway driving compared to Japan, and highway driving is said to be much easier on oil than city driving. Japan does a lot of city driving. I don’t think the engine will just flat out wear out any quicker just because 0w8 was used. I know those engines can still go 300k miles with 0w8, especially if the OCI was reasonable like 5k miles max.
Just my 2 cents