What is ATF?

Automatic Transmission Fluid. Normally used in Automatic gearboxes and in Power Steering systems.

Why would I want to put ATF in my gearbox?

To reduce drive train losses. If you look at my dyno you can see my 205 made 255bhp with 88bhp drive train losses, that's a whopping 34% loss of power applied on the wheels! To reduce these losses a thinner gearbox and/or differential oil can be used.

Gear changes will also become slicker and quicker.

What properties does ATF have?

Since using ATF can reduce drive train losses it must be thinner, this can be seen from the table below, and it is approx 50% thinner than a stock 75/90 GL5 gearbox oil.

Oil Rating Protection Viscosity
centistokes at 100o C
Stock 75/90 GL5 15 Maximum protection, maximum losses
TAF-X 75/90 GL4/5 14.4 Similar viscosity to stock fluid but reduced protection
SMX-S 75/85 GL3/4 11.5 A higher loss than ATF, but has a higher protection
ATF - GL 2/3 8 Least protection, least losses


Which oil is recommended by Toyota?

A GL5 75/90 gearbox oil. A GL5 has extreme pressure additives within the oil to help prevent wear. GL5 is specifically aimed at high speed/high load and hypoid gears; ATF is not.

Page 163 from the 205 owners manual


Note - the Toyota owners manual  makes it very clear that ATF is not to be used with a Limited Slip Differential application, the GT4 uses a LSD.

So, from the above statement I can only conclude that Toyota have obviously thought about using ATF in the gearbox, but have decided, (for whatever reason), that it is inappropriate. Why?

Does ATF offer enough protection?

Not according to Toyota or Castrol. The absence of extreme pressure additives can cause wear and/or pitting within the gearbox.

Can't I just change ATF every year?

No. The basic protection offered by a GL5 fluid just simply isn't there to begin with in an ATF fluid. Changing the fluid yearly will not raise a fluid's basic level of protection.

Loadsa people seem to be changing to ATF - they're not having a problem!

That maybe so. ATF may offer enough protection, but the protection required by the gearbox largely depends on engine power output, servicing, gear specification, gear design and gear materials. I do not have all the information regarding the gearbox specification etc, and I don't know anyone who does...

Filling with either Dexron II, Dexron III or Dexron III with additives makes little difference to the protection offered to the internal components of the gearbox, the basic extreme pressure GL5 component is absent.

How long have they had ATF in the gearbox? 100k miles? Is the gearbox still ok?

Yeah, but other cars run ATF and they don't have any problems or an automatic gearbox!

Supras run ATF when they come out of the factory...obviously their gearbox design is such that the gearing does not require a high level of protection...(and they're 2WD). Maybe the design of the gearbox/gearing is different, and so does not require such a high level of protection, maybe not.

GT4's are 4WD and therefore have the potential to generate greater forces/pressures within the gearbox, thus possibly requiring a higher level of protection.

My 2p

Personally I would not fill either the gearbox or rear differential with ATF. If the gears do not require a high level of protection then ATF may be a wise choice, if they do, then a breakdown and expensive repair will be required sooner than using GL5.

If drive train losses are an area to target then I would fill the rear diff with stock GL5 75/90, and the gearbox with SMX-S - this offers reduced drag and a higher level of protection than ATF...

I rang Castrol's Oil Technical people and quizzed them for about an hour, they offered the same conclusion as me...ATF may be very bad. Castrol can be contacted on (01793) 452222 for further information.

Why did Toyota specify a GL5 fluid? Surely they have enough R&D and know how and why a GL5 level of gearbox protection is required...

If a GT4 was capable of running ATF then why didn't it leave the factory filled up with ATF?

There is much conjecture about what oil is best - where is the evidence that ATF will provide a reliable alternative to GL5