T-5 Transmission Fluid Question


Founding Member
Mar 24, 2002
I've searched this forum and it seems everyone uses different fluids for their T-5's. I have a 1990 lx 5.0 with a 5-Speed ( I think that's the T-5, correct me if I'm wrong) and I'm about to change the fluid. The manual asks for mercon but people have suggested using Royal Purple Synchromax, GM Synchromesh, Redline ATF, Mobil 1 Synthetic ATF etc... So what's the best stuff to use for the smoothest shifts and best protection? Thanks!
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You want Dexron/Mercron III ATF fluid for the T5...that is the factory fluid needed.

When you see the GM Syncromesh it is not likely for the T5 but probably the TKO tranny...

Mobil 1 makes some good fluid...and D4 is excellent stuff from what I hear:nice:
A friend of mine that owns a shop in slatington PA, recommends regular atf dexron/mercon III, that's it. My buddy had 10psi s trim lots of other mods with stock t5, used reg fluid it lasted 3 yrs, beatin' the crap out of it nonstop. I, myself, run regular cheap fluid, i'm in my 4th year with my original tranny which has 146k on it. :D Better fluids are probably better, I guess, but I would stay away from additives.
Mustang5L5 said:
D3 is what you should be running.

Some guys get away with different fluid types, but the T-5 was designed for D3 ATF
Mike, you gotta watch it with the acronyms - at first blush I thought you were saying that Redline had a variant of Redline D4 that was for use in T5's. (I am being serious. for a minute, i was thinking 'Huh'). :)
I have recently heard a couple people say that regular ATF is better than synthetic, but I have a hard time with that.

I would go for any of the good name synthetics depending on what you like and what's available -- and cost of course. :nice:
89MustangGX said:
I have recently heard a couple people say that regular ATF is better than synthetic, but I have a hard time with that.
I agree with ya bud. with all the strange things people try in their WC T5's, it is hard to pick out the decent info (I am waiting for someone to try frying oil). a manual trans is not really stressing the hydraulic fluid like it would be in an automatic, so i dont think one can go too far wrong.

As you know, I am a fan of the synthetics. for the cost difference between conventional and synthetic (for 3 bottles) and how infrequent one needs to service the fluid, synthetic always gets my vote personally (matter of fact, your input last change helped me to stay with the D4 rather than try something else).
I just changed my tranny fluid an hour ago and motor oil poured out when i took out the drain plug. Now there's Dex/Merc III in there and it's like butter. Like butter I say. Now I understand what my short shifter was supposed to feel like all along. Used cars suck.