As it is "startup" which invokes the greatest wear due to engine oil starvation... what is your reasoning on this switch from 0W to 5W?

 

At the time I didn't know that 0w was actually Jaguar's recommendation and I referred to owner's manual.

Also, summers in Poland have been getting pretty crazy with temperatures reaching 30-40 Celsius so after reading a couple of threads here I figured a little thicker would be a better choice than 0w meant for temperate UK.

5w did also eliminate the need for any top ups.

 

Is it something established once and for all the 0w20 works objectively better for 5.0 N/A engines?

 

There is a common misunderstanding of engine oil weight.
The 0W component refers to "winter" (or cold) ambient weight at a "cold" engine startup.
The -20 component refers to the weight at standard engine operating temperature.

Therefore, as 30 - 40 C is lower then the standard engine running temperature, a lower viscosity is helpful. And the oil viscosity will "dynamically" adjust for rising ambient temperatures.
There are a lot of resources on this matter. A less technical document of value is here: https://www.castrol.com/en_us/united...t-is-0w20.html

And there is a lot misinformation. Be aware.
Cheers.

 

Can't say it's objectively anything but Jaguar did issue a bulletin stating the change?
My 2014 XJR has 5W-20 on the oil cap as this change looks to have been issued in 2018.

I have since changed to 0W-20 oil.
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AJ133 All markets Castrol EDGE Professional EC5 0W-20

 

I did a lot of research on this about 12 years ago then again about 6 years ago (when 03.5006 replaced 51.5122 per the bulletin), and I concluded two things:
The change from the earlier Ford WSS spec to 51.5122 (i.e. from 5W-20 to 0W-20) that happened around 2012 was for two reasons - improved fuel economy and slightly better (quicker) lubrication of the timing chains etc on cold start up. I would say 95% for increased fuel economy on the lab/bench test in an attempt to meet ever more swingeing CAFE and Euro "CO2 emissions" tests and thereby slightly reduce the resultant taxes and hence the asking prices for Jags. It is well established that a lighter weight (when cold) oil improves fuel economy, not a lot but measurable.
The change to 03.5006 was almost entirely to further reduce timing chain wear, together with ILSAC GF-6A replacing GF-5.
I have mentioned this before but I'm not sure I really need 0W-20 as I live in a relatively hot climate and it never gets anywhere near freezing in my garage so cold starts are not a worry. The main reason I have stuck with 0W-20 so far is ILSAC GF-6A and the fact that when it came out it was available in 0W-20 but not at all in 5W-20, so I stocked up on 0W-20 and still have plenty left. ILSAC GF-6A is now available in 5W-20 so when (if!!) I run out of 0W-20 I will change to 5W-20.
Last but not least I have never used Castrol oil in any of my Jags let alone the recommended Unobtainium, instead I have use Aussie brands Penrite and Nulon which meet the spec and most recently Valvoline, all 0W-20 fully synthetic, and I have never had a single engine problem.