2021 Prime Cold Start Rattle - Gen3 Prius Bad!

Similar problem = Cold start rattle.

 

Boroscope the cylinders.

4th gen's are prone to head gasket failure if Exhaust Heat Recovery (EHR) system springs a leak (which it's prone to), and it's neglected for some time, leading to one or more times where the engine had insufficient coolant.

 

I'm using the Walmart SuperTech 20K miles formulation, not the cheaper 10K mile one.

I just looked as my last jug of oil; could've sworn the oil blender/manufacturers' name and address used to be there - at least not on this jug. You may want to look at the Amazon & Kirkland jugs to see who's blending the oil for them.

Like you, I've got low actual ICE mileage on mine, but probably at least double or triple your ICE mileage when I do my annual oil changes. I usually fill-up every month or month and a half with over 1K miles on a tank of gas. The oil color is usually a very light honey brown, when it gets changed. Like I stated earlier, I need to use a tissue to figure out where the oil level is on the dip stick for the next 3 months after a change, because I can't see it.