Pulled into driveway and found a large oil puddle forming under engine. I discovered a small opening on the left side of the pan near the front corner. Whatever was in it had apparently fallen out. It seems to be threaded and there is a smaller hole just a bit further within it. I found this picture of a similar engine on E-Bay. The hole is visible near the bottom of the pan, just above the three parallel white marks on the floor.
I cannot find anyone who can tell me what fits into this hole or even what it might be for.
I cannot drive the car to a shop, and there in no point in towing it to a shop that does not know what is missing. (After many calls, I began to think I had gone mad and no such hole existed, but, happily, I found this verification that such a hole exists, but no explanation of what it is for..)
It's a '97 XJR with 80,000 miles.

 

Mystery solved.

 

Not a hole! Peril of searching under car with out a lift. Still not sure of leak's origin, but this "hole" isn't it....

 

By and large, we won't know much about the six in this subforum. This is for the X305 XJ12, not the X306 XJR6.