AW55 Auto box fluid changed to Dexron VI
 

I took my S40 for a gearbox oil change last week, just a sump dump and re-fill. The gearbox centre reckons they got just shy of 5L out. I was happy to hear that the oil that came out was in good order, however, I had changed 4L myself just 6 months ago.

Anyway, the mechanic wrote "filled with Dexron 6" when typing up the invoice. I was horrified at first having read so many threads about JWS3309 being the ONLY spec for this box and that Dexron (III) will kill it.

Anyway, the chap took some time to explain and further research confirms he was right in what he said. The Dexron ratings were originally devised by GM and are a sort of overall umbrella rating from what I can make out. GM lost the patent to Dexron 6 though some years ago, allowing other manufacturers to blend their own, so you must ensure it states 3309 compatibility on the spec sheet.

On a US forum I found reports of people using "Wall-mart" branded Dexron 6 in AW55 transmissions for 4-5yrs with no ill effect, in-fact a few posts suggest it has revived ageing boxes.

Dexron 6 is fully-synthetic and notably less viscous then 3309 to start with, however it is designed to lose less viscosity with age, so is more consistent over its lifespan. The lower viscosity also improves the pumping efficiency of the torque converter.

So far my box feels good. I've noticed it doesn't hang on to gears for quite as long under part throttle, which drops the engine back down into the peak torque range as opposed to revving out. This suits me fine and I assume is a result of the solenoids being slightly more responsive due to the new oil. The major difference however is that changing down in manual mode is immeasurably smoother. It used to to it with a lurch, now it just shifts down and revs come up to match with no lurching, very nice. There is no slip or flaring so the clutches seem happy, even under WOT with the remap.

Early days of course but by all accounts, the old Dexron III would kill an AW55 within a very short space of time. So, if anyone goes for a g/box fluid change and they mention Dexron 6, don't be too alarmed!