Gearbox Oil Selection
There are many many different grades of gearbox and differential oil. Toyota recommends GL-5 for the gearbox in all GT-Four models. Many disagree, but it is vital that a GL-5 is used, with good reason! Some use auto trans fluid with good results, it is now specified for most fwd Toyotas.
I have tried a few different gearbox oils in my eight years of ownership. The oil my ST165 came with was unknown so I changed to Pennzoil 75W-90 semi-synthetic. Despite this oil working very well in the W55 gearbox in my Celica Supra, it was not very good in the GT-Four, resulting in a notchy gearchange.
When my RC gearbox was fitted they used a Castrol oil. I assume this was Castrol Syntrac, as they suckered me NZ$35 a litre!! Despite the gearbox having just been rebuilt, the gearchange was crap! Very notchy.
I (again) changed to Redline oil, which at NZ$20 a litre was pretty reasonable. The correct oil to use in a GT-Four is 75W-90NS. This is the right viscosity rating, is a GL-5 oil and does not have friction modifiers for a LSD. The GT-Four gearbox does NOT have a limited slip diff in it! The viscous centre diff is a sealed unit and therefore unaffected by oil selection. This made a noticeable improvement to gearchange and was well worth doing.
GL-5 oil is mandatory in a GT-Four gearbox. As the excess traction of 4wd does not allow wheelspin the extra load is taken by the ring and pinion (the highest stressed parts in the gearbox). As a result of the high loads the contact surfaces of the teeth are known to wear prematurely. This was evident in my RC gearbox, which despite having new synchros had noticeable wear. We can only presume that the gearbox wore due to high mileage causing a poor gearchange. The owner used GL-4 oil to improve the gearchange, which increased ring and pinion wear. It was then rebuilt soon before I bought it. Hence use GL-5 or you will regret it.
That said, my RC gearbox was eventually replaced with a normal ST185 gearbox when the clutch failed. The gearchange in my RC box had deteriorated and I could trade the box over for only a $100 or so so I did so. I ran the new gearbox on ATF and the gearchange was excellent, who knows about the longativity though!
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