I have a question too. I didn't read whole post as it is very long, just the begining and I have just half of my answer so far. Cause you stated that 80W/90 is better over 75W/140. However....
Manual recommend 80W/90 GL5 oil. But I read many posts on internet, that 75W/140 GL5 is better and the only reason why manual does not recommend this one, is, that this oil didn't exist at that time yet. So some people think that 75W/140 is better, so why not use better oil.
According to my manual I should use exactly this one: http://www.ebay.co.uk/itm/Shell-Spirax- ... RTM1793562
and here is example of 70w/140: http://www.ebay.co.uk/itm/Castrol-Syntr ... RTM1793565
so now I have another question......is my differential Limited Slip type or Non Limited Slip type ?? Please can somebody tell mi this ? My car is Daimler xj40 4.0l engine, automatic, 1991. As you can see in description of 75w/140 oil, it is also suitable for Non Limited Slip dyff, so it seems like it is safer to use.
However I also found this: http://britishparts.co.uk/products/5421 ... e-JLM10814
and I HAVE it at home, I bought it long time ago with other parts, just to see what it is
but I am still not sure what is it for.
Is it perhaps special treatment for Limited Slip dyfferentials ? So if I have oil that is for Non Limited Slip dyff, than I should use this Additive ?
So will be Shell Spirax 80W/90 (which I don't know if it is for LSD or Non LSD dyffs) combined with this Additive good choice for my Jag ? Do you think I can mix it just like that with the oil ?