I have a small stash of GC 0W30, but my current vehicle is spec.ed for 0W20 or 5W20 (2007 Toyota 2.7L). The sump capacity is 6 quarts. I'm thinking 5 quarts of PP 5W20 and 1 quart of German Castrol. Any upside, or downside to this?
You should be fine with that blend. The most popular synthetic single quart blend here at BITOG is Red Line. The most popular conventional/dino for a single quart blend is Havoline.