Ask this on any Falcon, Commodore or Mustang forum and you will get six answers, four of which are wrong. It comes up in our inbox most weeks, so here is the whole thing in one place.
The short answer
A T5 takes ATF, and one 2.5 litre bottle does the job. Borg-Warner specified automatic transmission fluid from the first T5s in 1982, and Tremec still specifies ATF today. If someone tells you to put 75W90 in it, they are guessing.
The one we recommend and stock is KCK RE44 - a full synthetic ATF built specifically for manual gearboxes, held here in Perth. If you just wanted the answer, that is it. The reasoning is below.
Why a manual gearbox wants automatic fluid
This is the part that trips people up, and it is worth understanding rather than just taking on faith. There are three reasons.
1. The T5 lubricates by throwing oil around
There is no oil pump in a T5. The countershaft sits in the bottom of the case, picks fluid up as it turns, and slings it over everything above it. That works fine for the gear teeth, which are half submerged anyway. The problem is everything that has to be reached by oil in flight.
The worst case is the input shaft pocket bearing - the little bearing at the very front of the box that carries the nose of the mainshaft. It sits high, it sits forward, and it only ever sees oil that has been flung up there. Gear oil is too thick to make that journey. Put 80W90 in a T5 and the pocket bearing is the first thing to starve, which is exactly why we see so many of them come through the workshop chewed out.
It is not the only one. Each gear runs on a caged needle bearing between the gear and the shaft, and oil has to work its way in through the cage to reach the rollers. Thin fluid gets in there. Thick fluid does not, and you end up with every gear in the box sitting on a bearing that is only partly fed.
That is what it looks like when one lets go. The bearing has gone, and it has taken the bore of the gear with it - which turns a set of rollers worth a few dollars into a gear that has to be replaced.
These are the bearings in question. Note how tightly the rollers sit in the cage - that is the gap oil has to get through, and it is why fluid thickness matters more here than anywhere else in the box.
If you are not sure which pocket bearing type yours has, we wrote a separate guide on identifying your T5 input shaft pocket bearing.
2. The synchro linings are automatic transmission friction material
This is the detail that settles the argument. The OEM blocker rings in a World Class T5, first through fourth, are lined with the same friction material you find on automatic transmission clutch plates. Fifth was the only brass ring in the box. So these gearboxes want ATF for the most literal reason available - the friction surfaces inside them are automatic transmission friction surfaces.
The tan coloured ring is the OEM lining. Anyone who has had an automatic gearbox apart will recognise the material and the groove pattern immediately. The dark woven ring next to it is a carbon lined replacement.
A synchro has to grab. It needs a specific coefficient of friction to bring the gear up to speed before the slider engages, and ATF carries friction modifiers formulated to deliver exactly that against this material. Gear oil is formulated to do the opposite, to keep surfaces apart. Fill a T5 with gear oil and the rings slip instead of grabbing, and you get the crunch into second that everyone blames on worn synchros.
Half the "my synchros are gone" boxes we are asked to quote on have nothing wrong with the synchros. They have the wrong oil in them.
One thing worth knowing if you are rebuilding: the OEM lined rings are now very hard to find and expensive when they do surface. The carbon lined synchro kits we sell are the modern replacement - better material, better price, and our kits use a carbon lined fifth in place of the original brass one. Carbon runs happily on ATF as well, so none of this changes the fluid choice.
3. GL-5 gear oil attacks the yellow metals
GL-5 rated 80W90 carries a heavy dose of sulphur-phosphorus extreme pressure additive. That additive is corrosive to brass and bronze at operating temperature, and there are three things in a T5 it can get at: the OEM fifth gear synchro, which is the only brass ring in the box, the reverse idler bush, and the tailshaft bush in the rear extension housing.
This is slow damage rather than immediate, but it is real, and it is the reason a GL-5 gear oil is the worst single thing you can put in the box.
The four wrong answers
- "It's a manual, so it takes gear oil." The most common one, and the most understandable. It is simply not how this gearbox was designed.
- "GL-5 80W90 gives better protection." Better film strength on the gear teeth, yes. At the cost of the pocket bearing, the needle bearings under every gear, the synchros and the brass. Not a trade worth making.
- "Any ATF will do." Closer, but Dexron VI is noticeably thinner than the Dexron III these boxes were specified around, and the friction package is different. It will work. It is not what the box wants.
- "Thicker oil quietens my noisy box." This one is half true, and we would rather be straight with you about it. Heavier oil genuinely does damp bearing whine in a worn T5. It is also masking the problem, costing you cold shift quality, and starving the top of the case. If the box is noisy enough that you are reaching for thicker oil, the box needs bearings.
What about Castrol Transmax Z?
Transmax Z is a good ATF and it has done the job in T5s for years, including plenty that we have built. If you have some on the shelf, use it. Nobody needs to drain a gearbox that is running it, and it remains a perfectly sound choice.
We have changed what we recommend, and the reason is not that it stopped working.
RE44 is formulated for manual gearboxes specifically. A Dexron III fluid is designed around a torque converter and clutch packs, and it suits a T5 as a happy consequence rather than by intent. RE44 was aimed at gearboxes like this from the start, which is why TREMEC is printed on the label alongside LENCO, G-Force, Hollinger and Quaife.
It also comes in a 2.5 litre bottle, which is exactly what a T5 takes. One bottle, one gearbox, nothing left over.
What we use now
We have moved to KCK RE44. It is a full synthetic PAO and ester based ATF, hot blended in Australia, and it was formulated specifically for manual gearboxes that call for ATF rather than being an automatic fluid repurposed after the fact.
Those race gearbox names on the label are not decoration. The PAO and ester base holds its film when the box gets hot and stays shear stable through repeated hard changes instead of thinning out - which is exactly what a T5 asks of a fluid in a circuit car. KCK rate it for street, drag, circuit, rally, off-road and 4WD use.
It comes in a 2.5 litre bottle, which fills a T5 in one hit, and a 5 litre for anyone who would rather have fluid in hand for the next service too. We hold both here in Perth.
How to fill it
- Get the car level. On ramps or one wheel up and the level plug reads wrong and you will underfill it.
- Take the fill plug out first, before you touch the drain plug. It is on the right-hand side of the main case - the driver's side in a right-hand-drive Falcon or Commodore. Two reasons for that order. If the fill plug is seized you want to find out while the box still has oil in it, not after you have emptied it. And with it out the case can breathe, so the old fluid drains far faster.
- Drain, then refit the drain plug and do it up tight.
- Pump fluid in through the fill hole until it starts to run back out.
- That dribble is the level. Refit the plug. Do not be tempted to add more - overfilling a T5 pushes fluid out past the tailshaft seal and you will think you have a leak.
Fill plug above, drain plug below. Both take an allen key.
Putting the box back in after a rebuild
If the gearbox has been out and the shifter is still off, there is an easier way to get the fluid in. Pour it straight down the shifter opening in the top of the box before the shifter goes back on. No pump, no hose, no mess.
This is only worth doing when the shifter is already off. For a routine oil change on a box that is staying in the car, pulling the shifter out just to avoid using a pump is more work than it saves - fill it through the side in the normal way.
Two rules if you do fill it this way. The drain plug must be in and tight before you start. And the fill plug still has to be out, because that hole is the only thing telling you when the box is full. Stop pouring when fluid runs out of it.
How often
Road cars
Every 40,000km is fine. A street T5 is working comfortably inside what it was designed for, and there is no need to do it more often than that.
Race cars
Every third race meeting, or seven hours of running, whichever comes first.
That looks severe next to 40,000km, and it is meant to. Racing a T5 puts it well past the application it was designed for, and usually past its horsepower limit as well. Heat is what finishes the fluid off, and a circuit car makes far more of it than any road car - so hours are the useful measure, not kilometres.
At the end of every season the box should come out, be stripped and be properly assessed. A fluid change is maintenance, not inspection. Bearings, synchros, dog teeth and shaft journals all wear at a rate you cannot judge through a fill hole, and finding a marginal part on the bench over the off season is a very different proposition to finding it on a Saturday morning at the track.
While you are there: check the output shaft
This one is worth more than the oil change itself. From time to time, and ideally at every fluid change on a race car, pull the tailshaft out and check the output shaft for up and down play.
What you are feeling for is not the tailshaft bush. Play there is play between the mainshaft and the input shaft, right at the front of the box - and that is what breaks gears. Once those two can move relative to each other under load, the teeth stop meshing where they are supposed to and something eventually lets go.
If you find play, bring it in and we will re-shim the input shaft retainer. It is a quick job and we can usually do it while you wait. Very cheap insurance against a broken gearset.
Read the old oil before you bin it
Whatever comes out is worth thirty seconds of your attention.
- Fine grey glitter - normal wear metal in small amounts. A lot of it means bearings.
- Dark fibrous sludge - synchro lining material coming off the blocker rings. Shift quality is on its way out.
- Brass or bronze flake - the fifth synchro, the reverse idler bush or the tailshaft bush letting go.
- Black and smelling burnt - it has been cooked. Shorten the interval.
- Milky - water is getting in, usually past a seal or a breather.
And if you have been topping the box up rather than servicing it, the fluid is not the problem. The rear output shaft seal and the front input shaft seal are the two usual leak points on a T5, and both are cheap parts next to the labour of dropping the gearbox twice.
Still not sure?
If you are partway through a rebuild or trying to work out whether your box needs one, get in touch. We rebuild T5s here in Perth and carry the parts and the fluid on the shelf, so it is a quick answer rather than a sales pitch.