Here is the update on my 2004 XJ8. I managed to change my thermostat and get it all back together with no leaks. No new codes.After many test rides hoping that the codes would clear and the check engine light would go out, I cleared the codes and tried to redo the drive cycle . By some miracle, I actually have all but one sensor reset, with no codes and the check engine light is staying off. I would have gotten it inspected today but my mechanic friend was not in.
Now for the next worry! After I changed the oil, I've had a couple of instances where the oil lights stayed on for a few seconds after I started the car. And I've had a couple of times were the check oil light came on while I was stopped at a stop sign or stop light and had to rev the motor slightly to get it to go off. I know this is not a good thing, of course, but I am hoping I will be able to get the car inspected shortly and put a few miles on it and see how it's going to be. The car has over a 180000 miles on it so I won't be over surprised if the motor is worn and even needs to be replaced. The car is in good enough shape that this is not out of the realm of possibility, since I've seen some fairly low mileage engine and transmissions for sale at decent prices. I'd love to hear any thoughts anyone has on my situation, even sympathy lol
I'm editing this post once more to mention that today when I checked the oil after it cooled off I was about a quart low. I put 6 quarts in it when I did the change. I used Fram synthetic 5w30 and a filter with a check valve. I welcome any suggestions!

 

Assuming that you do have seven quarts in the motor. .
Find a spot to connect a pressure gauge, to check your oil pressure.

 

check the oil pickup for blockage

 

Another good place to look

 

Actually, there are only 6 quarts in there now. I'll add another before I do anything else!
I'll have to look and see where the best place to put a gauge is.

 

That makes sense.. I'm pretty scared to think about what might be blocking it though!

 

If the engine is in bad shape it could have low oil pressure.
With 180,000 miles should still be a good engine if it has been looked after.
The oil pressure warning could simply be a faulty pressure switch, I have had a one or two of those fail in various engines.
I note there is a pressure switch or sensor screwed into the oil filter block, easy to get at.
Worth checking, and it should be a generic Ford item.

 

What does the dipstick indicate? Don't overfill or underfill it, level should be mid-way in the hash mark.

 

I literally just checked it, and it was at the max line. It's on a slight inclined, so I'm moving it and I'll check it again after it sits a while.
meanwhile, I started it and the oil light went out as soon as it fired up.. its been running about 5 minutes and is warmed up, and no oil light so far.

 

I usually had to add some 7 quarts of 5W-30 engine oil to the crankcase of my 2005 Jaguar XJ8L as the oil filter itself would take some oil. Actually, I suspect that you are running the Jaguar with too little oil. Please check the oil dipstick.

 

it was a good thing I moved the car to check the oil. On flat ground it was almost a quart low. I don't know if that's my whole issue but I'm sure it didn't help.