I have 4 cyl, 5 sp, 1990 Nissan Hardbody with 90k miles. I switched to Redline MT 90 in the transmission this winter and the shifting improved (no more 2nd gear grind). My problem happened 2 weeks ago once the weather got warmer and I took a rare highway trip, the passenger side of the transmission as well as the underbody in front of the passenger side of the transmission was wet with transmission oil. It the area in the middle of the transmission leaking top down. The front (near) the engine and the rear (drive shaft) is dry. Actually the some of the oil sweep back to the drive shaft, but the rear seal is bone dry.
I have replaced the reverse light sensor and the rubber boot that covers the shifter shaft and top of the transmission (this boot can only be seen from the bottom of the truck). I still have the leakage (it is sometimes very wet on the passenger side of the transmission and several drops on the ground). This happens in city and highway driving.
I now believe it has to be the overflow/pressure value on top of the transmission (copper color metal tube, 90 degree bend facing out the passenger side of the transmission). Oil wouldn't splatter over the underside of the truck if the middle gasket was bad, there is even oil on top of a parallel suspension bar running along side of the transmission. I have checked the oil transmission oil level both hot and cold, and I am sure now it is not overfilled.
Using a q-tip, I plugged the overfill valve to test. It has not leak since my 15-mile drive to work. Could MT 90 be too viscous for my truck? Can I keep the overfill value plugged up?
Thanks for reading