Hello, I recently purchased a 1966 Unimog 404.1 radiobox and drove it from Texas to Cali without much issue.

The transmission always had trouble shifting into Third from a start and took some force to shift into Fourth as well. I also noticed that the truck would creep slightly in Third with the clutch pedal fully pressed. This creeping also happened in First while in reverse. So I concluded that the clutch was not fully engaging.

I would have to really push the shift lever to get it to shift into Third or power the engine off, shift and then restart. Fifth and Sixth shifted fine which is what allowed me to get it back.

Once I got it back, I noticed that the transmission fluid levels were at the lower limit. I added about a liter of Amsoil 75w-90 GL-4. This increased the pressure in the transmission and made the issue worse. Fifth and Sixth gears became hard to shift into and I would have to power down the engine to shift into Third or Fourth gears. Attempting to shift into Third would cause the truck to stall and the stalk would bounce off the limit.

I then adjusted the clutch pedal free play collar at the transmission and removed all slack in the pedal. This did nothing to rectify the issue. In fact it caused the transmission to be noisier when fully engaged though this might be attributed to the new oil.

Any suggestions on what could be the problem? This is the only issue with the vehicle.

thanks for any help! : -)