After I picked up the R last week, I phoned the service manager and asked him if ok to switch up from 0W 20 to 5W 30 as this very thin oil concerned me regarding long term wear in hot weather. Its pretty well known that a number of auto companies are specifying this thin oil to help meet the U.S.A. fleet fuel mileage future specs.. I'm told it gives them percentage of a mile per gallon in improvement. On another vehicle that I own, I got the ok from chrysler to go to 5/30.
Due to warranty concern, the Jaguar service mgr. contacted the regional service rep. who stated that I could be in breach of my warranty if it went ahead. The service manager said the thicker oil would not be a problem with the engine, as they specified thicker oil a few years ago, however the service rep. had 'made the call'... anyway, decided to leave it alone as an oil test as a result of an engine failure would detect the heavier oil.
I don't necessarily agree with the position taken by the regional representative, however, not much I could do at this point, (except for maybe writing to Jaguar head office.)
I'm not sure if anyone else has gone down this route of inquiry with the dealer and if so, I was wondering what you had been advised.
Lawrence

 

The Ford 325B spec oil is how they designed the engine and so i would stick with it TBH.

 

I'd give Jag the benefit of the doubt and presume that they've determined that the chosen oil spec is more than adequate.

Modern engines don't need the 30-40-50 weights we used decades ago.