I dropped the transmission pan and replaced the filter and gasket. The hose that connects to the left side of the transmission had to be replaced, so a new one was made up and installed.
Now that it is time to replenish the transmission fluid, I am at a bit of a loss in terms of what transmission fluid is going to be my best choice at this time. I am not abreast of the fluid advancements throughout the years that and would like some recommendations as to what would be the best fluid to use these days.
Hello A.J - what make, and model, is the transmission for your 1994 XJS 4.0L - if it is the General Motors Turbo Hydramatic 400 - usually referred to as the TH400, then you would normally use a Dextron ATF recommended for the GM models, but not the ATF for the Ford models - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 10/19/2020 1108hrs. EDT USA.
6-bangers use ZF electronically-controlled box
Well, I don’t know about it being a TH400. Wasn’t the TH400 in the 5.3? This looks like a ZF transmission but I don’t know how to read the numbers that I’ve photographed
The transmission was built for Dexron IIe.
You won’t find anything lower than DexIII now. It’s all backwards compatible. You can use any Dex 3 or higher.
If money was no object, I’d use a synthetic transmission fluid. Check out www.bitog.com of you really want to take a deep dive.
Dexron III and up is a synthetic transmission fluid.
What is higher than Dex III?
And
Is there some huge benefit to going higher?
Is this Mercon V supposed to be better than the rest?
I doubt it for day to day usage.
I believe theyre up to Dex6
Wow, there’s a lot going on in the world of transmission fluids, enough to drive you mad 
I think.
After hearing what you guys have said and a bit of internet browsing I think it’s probably best that I get out of the weeds on this subject. I wasn’t sure at first but now it seems that a nearly full 5 gal pale of Schaeffer’s 204SAT synthetic that I have stored in the corner of the shop will do just fine. Besides, I should have enough to replenish what came out for the filter change and drain it again, maybe two more times to end up with a complete fluid change.
Thanks for the help guys, I appreciate it.
May I allow myself:
https://www.xjsbanger.co.uk/XJS-auto-gerabox-service
Screw Dexron II. It’s like B/W TV next to the modern LCD. If something superseeds - it superseeds 
Above Dexron III there are fluids focused on completely different gearbox constructions.
The numbers of Dexron are just updates, like software releases. Each version replaces the earlier. None of them were bad AFAIK.
Dex 4 was all I used in my XJ40, same box.
Bravo Totally informative and highly entertaining
My 93 build 6cyl only has a 4 speed
His is the 4HP24 which is the 4 speed, I wonder if the ‘lock up’ is giving the appearance of a fifth gear?
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Janusz,
Are you saying stick to Dex3, since the fluids above 3 focus on completely different gearbox types?
That is what that link states, but they don’t seem to acknowledge Dex fluids past 3…
AFAIK the car has a 3.54 LSD rear end and yes, it is a 4 speed gearbox, but where does the impression of a fifth gear come from?
On the TH400 isn’t third gear 1:1?
I imagine it would be the same for 1:1 for the 4 speed?
for the TH400, Dex/Merc works fine, and you should NOT put synthetic ATF into a TH400.