I got my mom to put in extended performance Mobil 1 oil in her '07 Prius and we both agree the car has been getting better gas mileage. (near 48mpg at 75mph) Any other Prius owners notice fuel economy boost from high grade synthetic? I know one user on here was using Mobil 1 extended performance with a Prius.
Except Toyota calls for 5w-30 in the Prius, which could mean problems if you use xw-20 oil in the unlikely event of waranty issues. That said, I've personally used the 0w-20 Mobil 1 and 5w-20 Motorcraft in my Prius. I only noticed a real improvement with the Mobil 1 during the depths of winter. Not really worth the extra cost when following Toyota's maintence schedule.
If you plan to stick to the recommended oil weight, Mobil has 0w-30 now available in the big jugs at Wal-mart. Lack of the big jug for 0w-20 was the deal breaker for me. Cost not an issue, the german made Castrol 0w-30 is one of the few true synthetic oils you can buy over the counter.