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disguay, the bigger thing I would be worried about is getting an oil that has the correct oil certifications. If you look in the jaguar manual, it will state the specifications for the gear lube. As I recall it is only something like "AGL" (don't quote me on this). I have used Mobil 1 full synthetic 75W-140 in my car. I go with that because it is easy to find. But, other have gone with Royal Purple and Redline. Both of those are also full synthetic 75W-140 lube. So, look around, see what you can find.
The valve is a radiator drain valve with a collar on it to accept a tygon tube, then you have a 1/4 to 3/8 NPT reducer and then a 3/8 to 1/2" NPT reducer. You will have to shave the flats as the clearance is close. So, I would say to leave the flats on the 1/4 to 3/8 NPT adapter (like in the pic) and then you can use a thread sealant to lock all 3 parts together.
Did mine after I bought and drove it home to Utah from New Jersey. It was WAY ugly colored (@134,000 miles). See thread w/ pics here (used Royal Purple) =