Allow me to differ from your statement my friend. 10w, if you see the oil viscosity chart, shows that it is okay even for temperatures below 0 degrees. As also shown in the booklet, 10w can work for temperature as low as -20C.
It is more of an oil viscosity rating than the weather itself. For example, 10w40 shows that at minimum temperature, the oil's viscosity will remain a 10 grade (means slightly thicker than the 5w grade oil) and at normal engine operating temperature, it would remain at 40 (means slightly thicker than 30 grade oil).
If we also look at the oil, 10w40 feels thicker than the 5w30 (of course as the numbers show), but what I noticed was if you check the oil on dipstick right after driving the car for a while, 5w30 used to feel like almost watery like (i could see a very, very thin surface of oil on the dipstick) but 10w40 shows a relatively thicker coat of oil on the dipstick (when the engine is hot), which to me feels like better lubrication at higher temperature (i might be wrong, correct me please).
