Thermostat

couple things re prius:

maintenance message is mileage based, and has nothing to do with your oil level.

there is a low oil light, and a high temp light. if either ever comes on, your engine is toast.

so what's the takeaway? check your oil and coolant early and often

 

Agree that the oil pressure light is a "your engine is eating itself now" light.

The coolant overtemp light might be a smidgen less dire ... as long as it really is telling you the coolant is over temp, and not that the coolant isn't there.

If you see the light, a good thing to do is turn up the heat full blast. This might not be a comfortable thing to do, considering the weather you're most likely in if you're overheating to begin with, but it will help keep the engine cool until you get to where you can stop and investigate.

Unless ... you put the heat up full blast, and get blowing air, but it isn't hot. That tells you the coolant flat-out isn't there, and that's when you pull over at the next wide-enough spot and shut down no matter what.

That happened to me once (not in the Prius).

 

If you are wanting more gauges, you have several choices:
I use a Scangauge II, it is pricey but does quite a lot out of the box.
» ScanGauge II
There are about 100 gauges you can see on a Gen 3 Prius, 4 at a time.
I currently watch RPM, Intake air Temp in F, engine coolant temp in F, and instantaneous MPG, on my Gen 2 I worried more about Voltage.

If you have an old Android Phone you can use cheap software to make a customized gauge pack.
Torque — OBD2 Performance and Diagnostics for your Vehicle
You will need a cheap OBDII to Bluetooth adapter.
This can occupy more of your time, but is MUCH cheaper.

There are competitors to Torque, but I don't know them.

(Sadly oil pressure is a switch, not a gauge on a Prius, you can't read a value nothing is sending.)