He is full of sh.t!

Richard,
   I have run into owners of XJ40s who claim to have had problems with
plugs other than the Champions specified by Jaguar.  The problem with any
confirmation on something like this is, of course, the human error factor as
to whether the proper heat range was purchased or the proper gap set on the
plugs.

   You can use the standard plugs listed in the owner's manual which are
Champion RC9YC (RC9YCC in the UK)  for the 3.6 litre and 4.0 litre versions.
Gap is 0.035"  (0.9mm)  The 3.2 litre motor uses the Champion RC12YC
lug  -- same gap.

   Personally, I am using Bosch Platinum in my 3.6 litre and have for
50,000 miles with no problems. The only reason I use those is because the
parts store had a special on them!!

If you have concerns, simply use the Champions available at any parts store
for a couple of bucks apiece.

Cheers
webserve

I have not used Champion spark plugs for decades. My motorcycles did
not perform well with them. Also I have had more Champion stick in the
heads and gall the threads or worse. Perhaps they have improved but
the plugs I use now have never let me down.

I use nothing but NGK/ND plugs. Oh alright, I use Bosch too when NGK
is not available.

http://www.ngkntk.co.uk/

I laughed when I see these specialty plugs appear - "New, four earth
grounds or split grounds." NGK has made those for many decades, in
fact you can buy full earth grounds (Carbon foul-less) if you wanted.
Platinum? Tss NGK makes iridium!

I am not discounting other peoples experience, I am only speaking of
mine.

B.D.