Oil grade data.
 

March 1 to Sept 1 2012
Pennzoil conventional (yellow bottle) 10w30, ave was 42.58 mpg.

March 1 to Sept 1 2013
Same oil but 5w20, ave was 44.48 mpg.


Now, it is common for most people is to blow off the notion that grade can make a difference in the same oil. They just can't fathom being consistent enough in their driving patterns to see it! (Yes, this takes massive discipline)

Now, if we are a smart bunch, and hold to our belief that we are consistent enough to see changes, then I would like to hear your experiences regarding things that bring a small benefit.

Otherwise, why go through all the hassle if you are not seeing a benefit???

We may not always be using (all of) those small benefits towards ever better mpg.
We may be trading them off for speed / time and still get good mpg.
That doesn't mean we're not using these small improvements in the endless battle against drag.

I just switched my Corolla to 0w-20 synthetic last week as it's getting cold and helps me feel like I'm not hurting the engine by driving without letting it warm up. I hope I see a fuel savings

Hate to crap in yer Wheaties bowl, and again, just how I've come to see things, but after looking at my car's MPG over time, I now think unless you get a 10% or more bump over a large period of time, it sort of gets lost in the noise to say that any 1 change is going to be a significant difference.

There are just so many variables to what we get with our mileage that it is tough to say that this or that alone made the change.

Doesn't mean you're wrong, but it still is only half a chance of being significant to me to be able to say "This for sure improved my mileage"

I don't think you should be running such thin oil in such an old engine. Toyota's TSB EG018-06 recommends 5W-20W in certain '06 engines, and 0W-20W only in the '06 and newer 2AZ-FE engine used in the Camry, Solara, Highlander, and RAV-4. It specifically says not to run those thinner oils in older engines.

That TSB is worthless, Toyota denies the info in it, they tell you to go off of your manual. I contacted them on it. For example, the 1NZ-FE engine that's in my Yaris is on that list, but 5 years later, the manual still states 5w-30. Even the 2012-14 Yaris (1NZ-FE) uses 5w-30, yet the Prius C with the 1NZ-FXE, which is nearly identical to the 1NZ-FE (different cam profile and different pistons), uses 0w-20.

There is no difference between 0w-20 and 5w-20 at operating temp, the 0w is thinner than the 5w cold, but all oil is substantially thicker cold than at temp.

I'm ok with it for these reasons:
1) I trust Toyota engines longevity
2) I read the Oil Manufacturers Data Sheets to make my decision on viscosity
3) The colder temps have set in here in Buffalo
4) you get a bad feeling when you turn the key when it's 0 deg F in January
5) I currently have less than $800 into the car including tax, title, reg
6) I think you can choose an oil for your needs, race cars need thick oil due to heat, hyper milers barely tax their engine
As Sean says:

Also, just based on the oil viscosity data, the thread starters claim is reasonable knowing the oil is less thick, and that Auto makers are using thinner oils

They're your engines, but consider this: The bearing smoothness and tolerances on a 2006 Corolla or Yaris engine built for (and/or tested with) 5W-20W viscosity oil are likely to be much better than the bearing smoothness and tolerances for the 2000 versions of the same engines which were built for 5W-30W or 10W-30W oil. Also, we're not just talking about cold starts. We're talking about operating your warmed-up 2000 Corolla engine designed for 30W oil, hoping to lubricate it with much thinner 20W oil. If Toyota's engineers thought that would be OK, would they have said "DO NOT use these oils in engines other than those listed above. These low–viscosity oils cannot maintain lubrication effectiveness in engines with older designs and could result in smoke emissions from the tailpipe and/or unusual engine noise?"

"smoke emissions from the tailpipe and/or unusual engine noise" is engineer-speak for irreparable engine damage, IMHO.

Don't be discouraged by the worries of the thinner oils. Your car will have been replaced before that difference (if any) would show itself. My '95 Chevy 5.7 has 210,000 miles on it with 0w-20 iused in it since around 20k miles. My change intervals are 10k for a filter and 30k for everything. The truck doesn't use any oil between filter changes and has the probably never been driven without seeing full throttle. The truck will likely rust away before the engine dies....

30k between oil changes! I thought I was crazy running mine to 12k.

Have you ever had an oil analysis done after a change? I'd be curious to know the viscosity, contaminate levels, and TBN. That kind of data would be a goldmine to the community since it is very rare to see people putting that many miles on between fluid changes.