When I wanted to switch to synth on my RAV, I talked to one of the very knowledgeable guys at my Napa store. Not a "thinks-he-knows-it-all" type of guy either! As a friend, he gave me a piece of knowledge: he said each oil weight has it's own additives. Some additives are good in one car and bad in another. He wasn't sure at what point changing the weight would be bad, but advised me to stick to what the car calls for.
I did for the first oil change. But the second one, I decided to try a lighter weight. From 5-30 to 0-30. No differences from what I can tell. Lol, it would be a hard one to do ABA testing on!
Personally, I would recommend 5-30 synth, and a very good oil filter (Napa Gold or similar).
Even though I HATE shopping at Walmart, they do have the most economically priced oil I could find. Name brand stuff, so it's not like they can change the quality. I couldn't find the eco version of what I got in 5w30, but I do remember seeing it in the store. Here's what's running in the RAV right now:
Mobil 1 0W30 Advanced Fuel Economy Full Synthetic Motor Oil
And NEVER go with Fram filters!! (Bought at Walmart) They're awful!!