winter oil recommendations

Have you popped the hood and checked if the oil is actually low?

 

Lets assume you are low on oil.

You do not list a location which makes it hard to guess what Toyota recommends in your part of the World.
This will drag out every question you ever ask.

If you are in the US, 5W-30 is the correct oil (page 4 of this pdf) http://www.toyota.com/t3Portal/document/om/OM47509U/pdf/9.pdf

 

That is not at all obvious. First check the engine dipstick level before you reach that conclusion.
The master warning triangle light alerts the driver of a problem. Usually another warning light will be on or there may be a warning message on the MFD to give you a clue. Otherwise, it is up to you to figure out what the problem might be.

5W-30 is the recommended viscosity for North American Prius. Some owners have reported success using 0W-20 synthetic in cold weather, and that should provide slightly improved fuel economy. You run some risk that your engine oil consumption may increase with the thinner viscosity. At 52K miles on the odometer that should not be an issue.

 

First you should check oil dipstick. And correct it if it’s wrong. And if that was ok (or even if it isn’t) check fault codes with Toyota hybrid capable scanner. If you want to do it yourself get techstream. Post codes here and probably somebody can help you.

If where you live temperature regularly drops below -25c or -13F or ever drops below -35c or -31F you should use 0w-XX oil. But 0w-XX is going to reduce fuel consumption and engine wear in bit warmer temperatures also. 0w-20 can probably be used but 0w-30 is maybe bit better for engine (especially if you use same oil in summer). This also depends on specific oil.

 

The question should be: What does Toyota recommend and it is the same in winter as in summer unless maybe you maintain below 0 F.

And there is nothing obvious about either of your conclusions either.
The warning light you described is not necessarily about oil AND oil does not leak out faster when cold.

You need to visit a shop so they can hook up a code scanner and see what that little light is really trying to tell you.
And on the off chance there IS an oil problem, you should not drive it any more without first checking the oil level.

 

No, but please check the dipstick a tell us what you find.

 

Follow the manual on recommended oil types, and get the car inspected by the dealer or someone that knows Prius. Red Triangle could be anything, you need to see if any codes have been sent to the computer.