Use exactly the spec Toyota specify. If anything different was required they would change the spec.

How much difference has this change made to fuel consumption?

Most non hybrid Toyotas if you check the handbook are happy enough, depending on envisaged temp range, with anything from 5w 30 through to 10w 40, i haven't seen a 0W 20 mentioned.

Personally i buy good quality 5w40 full synthetics in bulk packs when on offer which is fine for all the cars we own for mixed temp driving, and comes under the ranges specified in each handbook.

Similar oil choice - different vehicle.

I have a Honda civic 2.2 CTDI. The original oil spec was a a fully synthetic 5W-30. The big H then brought out a revised oil spec of 0W-30 fully synth "in the interests of fuel economy"

So went out and bought Castrol Edge 0W-30 at vast expense and duly did the oil and filter change-ran it for a year and found that the economy didn't change and that the motor used a little oil between changes, which it didn't do before. So went back to 5W - 30 Elf synthetic which is readily available over here in France in the supermarkets at a reasonable price.

Result? If anything, slightly better fuel consumption and it doesn't use oil.

Conclusion- Don't really know!

I changed from 10W/40 to 5W/30 for a year at an oil change.

Made no measurable difference to petrol consumption (measured full tank to full tank).

Changed back to 10W/40 the next year, as it was cheaper and met manufacturer's spec.

A 5wXX oil is good for temps as low as -35c and I doubt you'll have that often in France.

A few years ago we had -10c in London but, it does not happen that often and I'd not base my oil choice on it. Although at the time the thickest oil I was using in my toyota was a 10w40 and the other car was on 5w40. So both were well within range.

When looking at the cold viscosity of an oil it is easy for people to confuse pressure and flow. You will get instant pressure with a straight 60 weight but, you won't have any oil around the bearings. In contrast using a 0 weight oil you'll probably have to wait a second for the oil pressure but, at least you'll have a flow of oil around the bearings.