Ask Dad

Pretty sure you can use whatever.

Don't use plain water like the other guy said. That will give you legionnaires disease.

Regular water can cause buildup of deposits and minerals which will eventually clog the jets that spray onto your windshield. If you're not going to use proper wiper fluid at least use distilled water.

Personally for three bucks I prefer the real stuff that causes water to bead off the glass and prevents bug splatter. Totally worth it.

Use what you want.

What you want to use is RainX. Or at least use RainX's additive to what ever washer fluid you do use.

If you live in a cold climate, just make sure the fluid you use won't freeze.

No need to match, but I like you use up what I've got before refilling just 'cuz. If you live in a place with particularly brutal winters (I've seen this in Wisconsin and Minnesota plenty, but less so in Colorado where I live now) there's a purple kind that's good for melting very light frost, and it'll better withstand the really crazy low temperatures. But in summer, I like the normal blue stuff to actually clean the windshield.