As we all know, opinions are like you-know-what............ everybody's got one.
Here's mine. In truth, these two fluids are NOT the same.
The Esso LT-71141 specified for the 5HP24 [non-supers] is a PARTIALLY SYNTHETIC fluid, that relies heavily on additives for life and performance; per ESSO's own data sheet. It is, however, still a very good hydraulic oil, far superior to conventional ATF's of the past.
The W5A580 Mercedes trans in the 4.0L supers uses a Shell product that is fully synthetic. [as does the ZF 6HP26 successor to the 5HP]
The idea they are the same comes from various fluid makers and their 'equivalence' charts. The Mercedes box specifies a fluid that meets their 'sheet' number 236.10. Makers formulate fluids and test them and they say it meets Sheet 236.10, therefore it is the same. After a number of years even ESSO added MB Sheet 236.10 to the LT-71141 specs. It is still a partially synthetic fluid that in my opinion, while still a good fluid, isn't on par with a fully synthetic product. Opinions differ, but Mercedes agrees with me. :-)
I have come to understand the value synthetics have over conventional fluids. They are much more resistant to oxidation, which is really the main reason fluid changes were needed with petroleum based fluids. These synthetics just keep going and going, without breaking down.
I'm with Mr. Hayden on this car, though. If the fluid was changed properly and with the proper fluid, I wouldn't hesitate a minute. An XJR is a fun car to drive.
Looking forward to your report after you have a bit of seat time!
Cheers,