Finally got around to r/ring Superblue’s plugs last night, and it was no easy task, in part b/c this time, unlike the first, I decided to use the “extended spark plug wrench” that comes with the V-12s instead of a standard 5/8 socket + long extension. The first time was “no problemo”, as I r/red the valve cover gasket (and half moon seals) at the same time, making “long reach” unnecessary for r/ring the plugs. This time though (and my not having a long extension of the 5/8 handy 
) I had no choice but to use the V-12 wrench. Oddly enough, the Jag factory literature seems to indicate this is the same tool to be used on the 4.0s, although it does not come with one from the factory unlike the V-12s did, at least in some parts of the world.
So, I approached the task, unfamiliar with the V-12 plug wrench (from Superblack’s tool bag, btw). I also kept hearing Palmdude’s words in my head, which were not exactly kind to that tool, from his past postings. 
Anyway, I eventually did get the plugs in and out, but feared at one or two points I had maybe damaged or stripped the threads in the cylinder head’s holes 
(aren’t plug threads made softer than the cylinder head’s, for that very reason ?) I think what was going on was that the V-12 wrench has like 16 points contact and not the more substantial 8-point of the 5/8 wrenches, allowing for some “slippage” when tightening up (or loosening) the plugs. And then there was the problem of getting the damn socket to let loose of the plugs when it came time to withdraw the wrench after tightening (I see now why the socket part is attached to the rod). Funny thing is that when the plug was finally loose, the socket wouldn’t adhere to them at all - I ended up using a pair of scissors (all I had at the time) to gently snatch the plug and lift it out of the well. 
Keep in mind the first time I changed the plugs I used quite a bit of “anti-seize” on the replacements. This time I used maybe a little less (had only one of those individual use packets this time for a spark plug r/r job). Anyway, this time I put in the OEM type Champion copper (plus) RC9YCs, after seeing several posters favor same on an earlier thread about the recommended plugs for our XJSes. The ones I took out from my first changing, less than 5K miles ago, were Champion 3344s, which are platinum. Before that, with about 20K miles on them, I removed the ones the PO had in her, which were NGK BKR6ESs, which are also platinum. Here are my observations as to the condition of these last two types upon removal:
1- General condition - both the NGK and Champions appeared in good/normal condition - not overheated (i.e. damaged, “baked”, melted) nor underheated (i.e. sooty, oily). Insulator near tip in both is white with very slight “reddish” tint which = fuel/additive deposits. The heat range for both types of plugs, btw, is a “9”, btw, which are fairly cool. As to the two, IMHO the NGKs appeared in less “used” condition, despite having 4x the mileage on them of the Champions;
2- “Coronal ring”. This is something there is a TSB out there somewhere on (Motorman?). The TSB advises techs that the appearance of this brown ring around the base of the insulator is normal and nothing to be concerned about, apparently caused by the direct proximity of the high discharge electricity to that part of the insulator, or some such. The Champions appeared to have a bit more of this ring than did the NGKs, the latter’s being quite slight.
The plugs I put in this time, the Champion RC9YCs, not only being copper rather than platinum, have a noticeably hotter heat rating than the other two types, being a “6”. It will be interesting to see how the condition of these plugs compare to the other two when it comes time to r/r them, esp. given Superblue’s issue of worn exhaust valve stem seals (i.e. blue smoke often on start up) - will the hotter heat range of the coppers lessen this condition and/or keep the plugs “cleaner”?. I do find it curious that on some vendor’s websites, on which I had my Jag’s info. entered, the seller advised that neither of the earlier types of plugs “fit your '94 XJS 4.0”. Not sure if they were referring to size, as the thread and hex measurements are pretty much identical to those of the Champion coppers, or the fact that they are platinum vs. copper. ? IIRC, some of our members reported that copper should be used in our XJSes and NOT platinum, based on their own experiences. btw, so far, I can’t really tell any difference as to running with the coppers, but my MPG does seem to have increased a bit. 
). I guess it’s just another “weight saving” measure … ? 
At first I tried inserting the new plug into the socket and then quickly pushing it down the well, but when I tried that the plug would always fall out of the socket before making it to the hole. 
. Must be the latest plug technology.