Engine oil question

Toyota's 5W30 is fine. Mobil 1, Pennzoil Platinum and Amsoil synthetics are fine. 5W30, 0W20 viscosity are fine. Several other choices are probably fine also, but have not been as well researched.

Depending on the completeness of the old oil drain. 3 1/2 quarts will bring it up to mid-stick or the top mark. Anywhere within that range is fine.

Most of the oil discussions we have here (brand, viscosity, change interval, analyses) are directed at enthusiasts who want to achieve something a bit out of the ordinary. I'm guessing instead that you're just regular people, wanting to drive the car. So, relax.

If your Prius has a big odometer and you have been using non-synthetic oil on 7500 mile intervals (as was the specification in 2002), then you probably have some engine varnish on the non-contact surfaces. If you change to synthetic oil now, some of that stuff is going to move into the oil. Therefore I'd suggest the first synthetic oil change to be on a short interval, like 2000 or 3000 miles. Then it'll be shiny inside again and you can revert to whatever oil change strategy you like.