[GaKu]

I was averaging about 8.9l/100 km in pure city driving after I got my car serviced and got new tyres on. This was a bit high but I blamed my new Michelin crossclimates for it and just thought..well thats life, you cant always win.
Recently got the oil changed and suddenly noticed that the computer now reads about 7.8l/100km. I am driving the same routes and this was very surprising but the reason was very simple - oil viscosity.

I had 0w30 earlier since I read some forums where this was recommended for cold climates and thought good, this gives me better cold starts. I had also misread the manual which does recommend 0w30 but for diesels.
Now changed to 5w30, as recommended by the book and behold - its a new car. I never thought this would matter so much but I was so wrong. (You could read my other thread on engine rattle which was also due to the wrong viscosity)

So in the end, all I am saying is, I should have gone by the book 

On a recent long run, with the AC on and tyres upto 2.4 bar, I averaged 6.9l/100km. I dont think it can go better than that unless I drive like a nanny.

Sure it's just not the different weather conditions and trying to achieve better economy that has made the difference both of the oils you used have the same operating viscosity so milage should be the same, it's only the cold/starting weight that has changed and the slightly thicker oil at cold start is better at eliminating noisy lifters and tappets as you discovered, I would think in the winter months a 0W-30 or better still 0W-20 would be more suitable for your climate...the 0W-20 would give better mileage being thinner at operating temp.
Oil brand and quality can also be a factor in economy like with synthetic's to mineral oils.

My own oil I use is a Penrite 5W-30 PAO (Poly Alfa Olefin) true synthetic oil which operates like a 5W-20 oil for better economy though has the protection benefits of a higher ambient temperature 5W-40

[GaKu]

8.9 was the number I was getting all year round and my driving style is still the same. And I was also of the opinion that operating viscosity would be the same but my experience says otherwise.... Unless the mechanic screwed me on the claimed oil the last time.
BTW, The lifters were noisy at all times, not just startup.
Both oils were fully synthetic. Castrol ti 0w30 and now shell hx8 5w30.
The ambient temperatures for 5w30 as in the book cover the full range here and some more so that doesn't matter I think. In fact the book says the 0w30 doesnt cover the summer temperatures here. I also ran an engine flush the last time but its hard for me to believe that has made such a big difference because I used to get 6.4 on highway driving prior to having 0w30

At least you're now getting better economy and a quieter engine it would be nice if it dropped down to the  6.4 like you used to get

[GaKu]

At least you're now getting better economy and a quieter engine it would be nice if it dropped down to the  6.4 like you used to get

I'm quite optimistic about hitting that number. But I'll hate my driving style while being overtaken by tiny cars.