 Transmission fluid leak from behind instrument cluster
I also have this problem. First, I had the transmission fluid leak out the speed sensor. And the speedometer cable broke so I put a new one in, but still leaked out the base. I took the car to a transmission shop where they replaced the speed sensor O-rings with old but better ones off a sensor they had there. The large round one and the small one in the top where the speedometer cable goes in (and the special circlip holds the cable against it). This didn't stop the leaking at the base of the sensor. I took it back and they said I should just replace the speed sensor. I replaced it with a new one with brand new o-rings. I refilled the transmission. And everything was fine until I got the car up to highway speed and it started leaking out of the speedometer housing under the dash. Like tk_tabler's theories, there is a correlation between speed/heat and leakage.
I have an added possibility, the manual says to replace the fluid with automatic transmission fluid, not 30w oil. The tranmission shop told me they always put 30w oil in the case and never tranny fuild. I never had a problem until I replaced the clutch 5 years ago and that's when it started leaking out the speed sensor base in small quantities. With the new speed sensor the o-rings are tight so it can only auger up the cable and dump out the speedometer. All the mechanics I talked to had no clue of the problem. I want to drain the tranny fluid and put the thicker 30w oil in with the hope that the thicker oil will not auger up past the o-rings as it is too dense unlike tranny fluid . Does anyone/mechanic have the real answer to this problem? | 
