I took my 2020 Yaris in for 45,000 mile service. I asked them to check for leaks in the cooling system when they did the service, as I noticed the coolant level was low in the overflow bottle when changing windshield wiper fluid.

When they finished inspecting, they said they didn't find any signs of leaks , but recommended topping off the coolant in the reservoir. They said they didn't have the type needed for my Yaris in stock, but could have it in by tomorrow.

I asked if I topped it off myself with coolant from Autozone that was compatible, if it would be okay. The service manager recommended not mixing different coolant types. They said while it might be safe for my engine, third party coolants tend to contain different chemical makeups that when mixed with the OEM coolant, could cause issues that could damage the engine. They said I would essentially need to flush the old coolant out, and refill it with the coolant from Autozone if that is what I wanted to do. They then went on about how it was better for a professional, such as them, to flush the system as it is sealed off and doing it yourself could introduce air into the system.

My question, is this true that I would be better off waiting until tomorrow for OEM (which costs quite a bit more), or would I be fine finding a compatible coolant and topping it off in the bottle?

Thanks for any help.