Why does my car downshift on its own when going up small hills. It didnt do this all summer but has this winter, living in Minnesota

No it does not have an external fuel filter,downshifting going up hills sounds normal and the best way to check a cat is with a gas sniffer,or a scanner that monitors both pre and post o2 sensors

first we have to know what make, model and year the car is. is it a 4x4 or all wheel drive
comment with the details

It will lack lots of power going up hill, especially if its a really steep one and you keep it in drive. Make sure you downshift to second, or fist if its really steep.

Perhaps a bad Thermostat

check air filter and run gas line anti-freeze

Most likely 5W-30. The W is for winter. The numbers tell summer and winter. A quart of this oil will act like 5 weight (very thin) in the winter and 30 weight in the summer (thicker). Let's not go in to how that is done.
If you live up north, a 5W-20 would do well also. Down south 10W-30 is often used.

Doesn't this car have an onboard dash light to show Winter Mode? Sounds more like it's an engine or transmission issue. Tune up? Trans service?
Is the CEL (check engine light) on?

GL,
Td

Have your Catalytic converters checked! Sounds like there getting plugged!

Standard 87 octane is all that car requires. You will gain no benefit from using mid or top grade fuel.

Most passenger car tires are run between 32 and 35 psi, regardless of season. You can look at the side of the tire to find what the max cold psi pressure is. You will normally find that it is 35psi.

As for summer/winter tires. If the wobble only happens when the snow tires are installed, and not with the summer tires, that obviously eliminates the car as being the problem. Do you have your summer and winter tires mounted on different sets of wheels, or do you swap the tires every season on the same wheels?

If they are on different sets of wheels, then the winter tires are out of balance, have some sort of damage to the carcass of the tires, or you have a bent wheel. If you are using the same wheels and swapping the tires every season, then you need to have your winter tires balanced and see if it solves the problem.

Yes your car does have Pistons.. and No its not normal for this to happen in the winter. However I would recommend that you use a lighter oil in the winter than the summer like 5w-30 or 10w-30 no higher. Good Luck