Toyota has often recommended 10,000 mile oil changes when using synthetic 0w20.
Amazon has single 5-quart jugs of Ultra 0w20 for $32 each, Amazon Prime free shipping.
I'd go with Walmart's Pennzoil Platinum (not Ultra) 0w20 for cheap, IF you really want to stick with Pennzoil.
Certainly any 0w20 you see on the shelf would do, but you've got a lot of miles on that aging Scion, so I'd recommend the Walmart Valvoline Maxlife full-synthetic 0w20 which meets dexos1 Gen2 and ACEA A5/B5, and the "High Mileage" formula has some extra seal conditioners to keep those old seals from springing leaks in the future.
I do recommend using a 0w20 over a 5w20 for below zero temperatures. There is a small amount of difference in flow at below zero between the two.
No problem with changing brands of oil.