Brother , kindly do your research aswell but let me state some hard facts...
Your car is a 2008 model. Old car , old engine. Your engine has more wear and tear.
now coming to the oil. 10w or 0w or 5w.. Doesnt matter. This viscosity is for your cold starts. Majority of your engine wear and tear occurs during the cold start up.. since your engine is old, u need a slightly thicker oil to prevent further wear and tear.
So 10w is fine !!
Now coming to the 20s, 30s ,40s etc.. You tell us that you travel a quite lot. You need a thicker oil to work those pistons in the long run. Yes lower viscosity will give u better mileage, but will increase engine wear n tear. Gaps ajaenge in your engine moving parts.
So 40 viscosity is absolutely fine for you!!
If i have a 2012 7000km mileage Passo nd i drive like 15-20 kms daily with atleast 2 cold starts a day, then 5w30 is perfect for me.
But for a 6 year old high mileage car, it isnt advisable.
Hope you get my point