I think you'll need to be a tad more specific in what you're after. This is an extraordinarily broad topic.
I changed my oil for the first time and used Mobil 1 (can't remember if it was 10W-20 or not. Whatever is recommended) And I have gone from 47mpg to 52mpg. Of course that could be due to the change in weather as well, but I have been running the AC more often than not with no adverse effect. So I guess the jury is still out.
I also changed to Mobil 1 with my 5K service. I don't know how you tell impact on MPG with so many other factors: break-in period for car, tires and driver; weather; tire pressure; winter vs summer gas blend; route selection ... You can see my MPG history in my signature. Good luck, Gabe
If you get an increase, and you probably will, it would likely be no more than 1 mpg, which would be hard to notice.
If you use your vehicle for frequent short trips with little to no warm up time you will see a modest 1 - 3 MPG gain with both synthetic and a thinner weight like 0w30 or 0w20. The real benefits of synthetic oil will be less wear and tear on your engine and after you get past your 60,000 mile powertrain warranty period you can do extended oil change intervals.
Change the oil yourself because if you keep your eye open you can usually get Mobil 1 or Castrol Syntec at a price of about $4 - $5 a quart. With the Prius you only need a tad more then 3 quarts. That adds up to about an extra $5 for an oil change well worth the price especially when most synthetics are designed to go 10,000 + miles.
Pennzoil Platinum is really good stuff just buy the 5 quart jugs for 23 bucks, or buy seperate bottles. Once you open the 5 quart, only roughly 4 quarts goes in..so for 6 months that other quart will go stale maybe since it was opened. Syn really honestly is over kill on this motor I have come to think. IT wont hurt anything though. if you want to save a few bucks go with a synblend like motorcraft or Castrol. The motorcraft is a great bargain and is good oil. Pennzoil Plat is a Group IIIbase stock just like all the rest of the mass market Synthetics, Mobil 1 used to be a Group IV which is a true syn, but they stopped that formula recently. The only TRUE synhetic from mobil 1 now is the Extended life oil which cost more. So in short just get any of the common Synthetics, they are mostly Group III's now. or just run cheapo dino oil, even most of those are good for 5000 miles anymore.
