has a chart which shows recommended viscosity for various ranges of temperatures.
My car doesn't run in storage in the winter so for me the 0W part of the 0w-40w is not an important consideration . The car returns to the road in April for the summer into the fall. Here in New England summers are normally in the 70's with mid summer (July-August) temps in the high 80s low 90s with some heat wave days in the mid to high 90s. To me , this is where the 40W "covers" those high temps according to the chart while the 30W reaches in the 60s on the chart.
I read or heard somewhere that the reason Jag likes the 30W is that it provides slightly better gas mileage. Who knows ? I realize that the cooling system when operating properly should keep a constant temp. However, I just feel a little more comfortable with a little higher viscosity oil on a 95 degree day.
For those who have not seen the owners manual oil chart, I'll try to snap a picture of it and upload it and you can see what I'm talking about.

 

Here's my argument why 0w40 can't possibly do any harm. Firstly, I'll repeat the viscosity specs :

Mobil 1 0W-40 Value
Viscosity, cSt (ASTM D445)
@ 40º C 75
@ 100º C 13.5

Mobil 1 5W-30
SAE Grade 5W-30
Viscosity @ 100ºC, cSt (ASTM D445) 11.0
Viscosity, @ 40ºC, cSt (ASTM D445) 61.7

So if you are using 5w30 the viscosity is somewhere around 11 cSt at operating temp. That's a factor of over 5.5 variation vs. the 61.7 during warm up. So this demonstrates that the engine can handle a wide range of viscosities.

If you are using 0w40 the viscosity at 100C will be 13.5. So is anyone seriously claiming that the engine can tolerate between 60 and 11 cSt but will be damaged by 13.5? Makes no sense.

All that is happening with Ow40 is that you are getting a tiny bit higher hot viscosity, and a little more protection, at the expense of a tiny bit more fuel economy.

We are not talking huge differences here. They are tiny.

 

That's because of the '0'W... part of the rating. Make life simple, buy a 5Wxx or 10Wxx oil and you're all set.