Following on from my other questions:
The owner's manual says 0w30 is only suitable for tempratures between -30C to 10C.
I've seen the GC 0w30 mentioned as suitable for the CRDi.
But given that Australian temperature often go well above 10C, is it actually suitable?
And yes, I'm a little puzzled that 5w30 is suitable up to 40C (according to the owner's manual), but 0w30 is only usable up to 10C ...
I would be so bold as to say whomever wrote the owner's manual oversimplified it.
A 0w30 and a 5w30 (or a 10w30 for that matter) could all be the same hot viscosity, as you wondered. They could all vary by quite a bit too because the viscosity given is not a single viscosity but instead a range.
The GC (German Castrol) 0w30 is in fact
almost identical to Mobil 1 0w40 in its hot viscosity. It's just that one is slightly below the crossover between 30 and 40 and the other is slightly above it so determining the different sticker numbers. Probably not by accident either, Mobil might have thought that a wide 0w40 (and theoretically difficult technically) number was easier to market and Castrol might equally have thought that a 30 number covered more vehicles or implied better economy or any number of marketing reasons. All supposition of course.
So, a 0w30 or 5w30 is ok so long as it's not a thin 30! (my qualification). Equally, I think some 40 grades are unnecessarily thick.