Users of this group should head over to the premier Jaguar forum and discussion groups at....
  http://jag-lovers.org/
It's by far the largest and most lively Jaguar forum on or off the net.  It is completely
searchable and one can find answers to most technical questions.  The synthetic oil question has
been discussed to death and you'll find opinions from extremely knowledgeable and experienced folks.
There are separate forums for most Jaguar types.
  See ya'll there!

Depends a great deal on what you drive and how you drive it. Most
enthusiasts in Europe uses synthetics as a matter of course in any
performance car. Certainly you should always use it from scratch in a
new car which is going to be driven hard. On the other hand, if you
have an old car which is a bit loose and burns oil - as early XKs were
exptected to, for instance - then no point in burning or leaking
expensive stuff. All the guff about synthetics not being suitable for
certain types of car is just old wives tales. Might have been true
once. Isn't now. Never had any problems switching an older car onto a
synth or semi-synth oil. Looks expensive on paper, but then you don't
have to change it so often.. I stick to an annual oil change on my
(relatively) low-mileage cars.

David Betts (davidb@motorsport.org.uk)
The Classic Car Gallery: http://www.ofoto.com/I.jsp?m=17830847103&n=398038677

Thanks for the input / responses.
 I've been an avid fan of "Jag Lovers" website for years, and believe it
to be the most exhaustive collection of info available for "cat" lovers!
thanks again, rogerj  (aka Leepincat)