One of THE most common questions is "What kind of automatic transmission fluid do I use?" so let's start there and get it over with! That's probably why you turned to this page anyway.
If you have an XJ6 your owner's manual and the shop manual specify "Type G" trans fluid. That's where the problem begins; "Type G" is a European designation and may be obsolete by now even in Europe.
The correct fluid to use is "Type F" or "Type FA". These are readily available. If you prefer a slightly softer shift you may use "Dexron" transmission fluid with no harmful effects; some Jag owners prefer this.
Incidentally, the Borg-Warner transmissions used in the 6 cylinder cars (and older V12's) is nothing exotic, it's cousins dating back to 1950's Fords and Ramblers. It is not a particularly smooth or refined transmission but is robust and reliable.
All Series III 12-cylinder cars (and later Series II V12's) use the General Motors Turbo-Hydramatic 400 transmission. The correct fluid for these is "Dexron", period. The latest formulation of this ATF is "Dexron III" and is what you should look for.
This transmission, by the way, was first used by GM in 1964 and became a industry benchmark by virtue of it's ruggedness and smooth shifting.
Routine Transmission Service Procedure
Firstly, check the fluid level on a regular basis and top-off as needed. If you're a quart low your transmission will probably tell you about it by giving sluggish shifts and delayed initial engagement.
The major consideration in transmission maintenance is changing the fluid. At minimum the fluid should be changed every 30,000 miles but many Jaguar owners prefer to cut that interval in half. There are two schools of thought on changing your transmission fluid:
The standard, conventional method of "transmission service" is to remove the oil pan, clean it, replace the transmission filter and pan gasket, button it all up and top off the fluid. This is fine except that dropping the pan only removes about 1/3 of the total fluid, the balance of old fluid remaining inside the transmission.
The newer (and fast becoming popular) method is a "transmission flush". This method uses a pump to pull out all of the old fluid and replace it with new fluid. No old fluid is left inside the transmission. Some clever Jag lovers have devised their own method of doing this at home but, really, it's easier just to have it professionally done.
One common question about the new method is "What about the filter?". True, the filter would not normally be changed when a flush is done but that's OK. The flushing cleans the filter and, truthfully, if your trans filter ever actually clogs up then your transmission is beyond the point where new fluid will help. As an alternative, do both types of service to begin with and then just use the flush method on future changes.
You'll want regularly inspect a few things on your transmission. You'll probably find leaks. Transmission leaks can be a real nuisance. Some are easy to fix, others difficult. You'll have to balance expense vs. benefit. Check the shift cable for binding or deterioration. Check then pan for dents or other impact damage. Look at the cooler lines, checking for loose fittings and clamps and deteriorating rubber hose sections. If transmission fluid hoses are due for replacement, you must use hydraulic hose, not fuel hose!
If your shift cable is binding and/or needs frequent replacement you'll want to check that all of your electric grounds are in good shape. If they are not, then your shift cable becomes the ground and goes to heck in a handbasket... in a hurry!
The Borg-Warner transmission does have a band adjustment, but that job is beyond the scope of this article.
Clunking with the Borg-Warner transmission can often be eliminated by adjusting the downshift cable. Not difficult, per se, but a delicate hand is required. Post on the Jaguar Forums for more information.