Cite on synthetic being helpful for:
midlife engine
stop-and-go driving,
excluding sites from oil blenders or marketers?
Point conceded for low temps, so long as low temps are defined as under -20F.
On edit:
Oh yeah, if you’re not doing it already, I suggest running 0W30 in this vehicle during the non-summer seasons.
A cite shouldn’t be necessary - synthetic should help prevent gunk buildup in the engine, and on an engine with 120k miles, this is important. And yes, it will impact mileage.
Punishing this engine as much as the low temps are the short runs - consider the environment here. This engine starts cold and seldom fully heats up. If you think synthetic would be helpful just for the temps, this is compounded by these other conditions.
Can I play?
Synthetic? IMHO a great idea. Toyotas have been known to sludge, and synthetic will help keep that in check. BTW sludging is worse in cold climates. So from where I sit in sunny California synthetic looks like a good idea.
What weight to run at what temp. The answer to this is simple. Your owner’s manual has the correct information. Period. No one you meet on a message board will know more about the engine in your car than the engineers that work for Toyota, designed it, and put the oil specifications in the owner’s manual. I include myself in that group of people that you come across on a message board. 
Nunavut Boy If you T-stat is bad, the heater will blow cold air, if it is only partway bad, the air will still be warm. Let’s say your thermostat is supposed to keep the coolant at 195F. It goes partway bad and the coolant only gets to 145F. You will still have warm air from the heater, although you might have to move the lever a little further into the red to get it. But the computer will see that your engine is not hot, and will add extra fuel. MPG goes into the shiter.
That actually totally explains a problem I’ve been having since I had a blown radiator replaced. The engine won’t reach the normal running temperature, and the heating won’t blow anything but cold air on cold days.
Thanks for the reponses everyone. I think the cold has more to do with it than anything. The weather here has been a lot warmer than it has been and I’ve managed to get 80km out of a quarter tank so for. I’ll keep watching it as it warms up, but I suspect it’s the damned cold.
I will also change to synthetic oil when i fix the power steering in a couple of months.
Change the thermostat, it should not be that hard, or that expensive. You should see a pick up in mileage right away. At 9MPG, it won’t take long to recover the cost of the t-stat.
I’ve read gobs of data on this and never seen any solid evidence to support the two claims I demanded cites for… hence the request.
The lower end of 5W30 working properly is at -25F.
You’re right, though, you should listen to your manual on these things, especially with regards to the number to the right of the W. I would never advocate messing around with that.
If the vehicle sat for a while before you bought it, the fuel filter may be partially clogged with gunk from the gas tank. Changing the fuel filter shouldn’t be that difficult or expensive and should help the gas mileage improve.
Well, I just wanted to come back and report what happened. It seems that with my Mazda 3, during the winter months, even when there’s no snow or if it’s not particularly that cold out (just above freezing), I get somewhere in the range of 25-27 mpg highway. I just got back from a trip to Quincy, and I averaged just over 32 mpg down, and am on course for about 34 mpg back.
At any rate, it seems that for me, a 7-9 mpg swing between winter and summer mileage is normal. (This is all based on highway mileage).