Does anyone have any evidence as to whether or not constantly using the manual gear selector effects the longevity of the transmission versus just leaving it in "drive"?

 

Evidence? The premise of your question would require that a controlled test had been performed, where 100 (or more) drivers were told to use the automatic option only, and another 100 drivers were told to use the manual shifting option exclusively. At various intervals, the cars would be examined for transmission problems or repairs. This test would have to go on over a ten year or more time frame. I seriously doubt whether or not anyone has performed such a controlled test. Anecdotal evidence says that an automatic transmission prefers to choose its own shift points, and that forcing it to do otherwise is not the optimal operating procedure. I would suspect that the transmission in the Jaguar would be the same as others in this respect. It probably won't mind too much if you do it on occasion, but it you do it constantly, I would suspect that it may not respond favorably. I have had other cars with automatic transmissions that employed fuzzy logic, meaning that they adjusted their shift points to the habits of the driver. I do not know whether or not the transmission in our XK8/Rs employs fuzzy logic.

 

I believe it does, fussy logic that is. It will also learn very quickly to hold a gear on a very twisty road. This was in an article I read somewhere and applied to the 6 speed box. Haven't tested it though !!

 

It appears to "adjust" to our driving habits. When I received it at first it started out smooth and shifted lazily. Now it seems to launch and wind out more before shifting.

The ECU at first also told me I had well over 400 miles range on a fillup. Now it tells me I have 315 mile range on a fillup.
(Does that tell something about my driving habits?)

On the topic, I seriously doubt manually shifting will cause any damage or excess wear on my experience of doing so for decades in other automatic cars. Since it means you want to wind it out more and maybe also motor brake with it, you'll be working the drivetrain more, but of itself there's nothing wrong with that. Gas miles will drop is about it.