Transmission shifting

I have a 1959 220s with a three speed manual transmission. The transmission shifts very smoothly when the car is cold. However it becomes very difficult to shift when its warmed up especially the reverse. A mercedes dealership has changed the transmission fluid to a automatic fluid. This has not helped a bit. Can you please let me know what could the problem be.

Thanks

[j9fd3s]

if it has a hydraulic clutch that could be it. i dont know if the pontons have them or not though

mike
69 280s <---2 pedals
66 250s <---3 pedals, hydraulic clutch