I have a1999 September xj8 sport,,,,it has nearly 99,000 miles on the clock ...i was told that the cam tensions ect  needed to be changed as they give lots of problems...i removed the cam cover on the left hand side and found no type of damage any where ....the car has always had a yearly oil change and sometimes was only 2000 miles max!!!    the car has hardly been driven over 70 mph in its life  .. could the regular oil change and low top speed be the reason the parts are not damaged ???

I believe that year used plastic in the tensioners.  Later years used metal to make them more durable.  In your case, age may be as relevant as miles.