Don't know details of this transmission but likely no synchromesh on reverse. Not normally a problem as the gears are rotating slowly at that point. Always select when stationary to give it the best chance. Historically with old gearboxes it was common to select 1st and then reverse if it was reluctant, especially with cold oil. This might work for you. Mileage means that it's likely worn to some degree. I would change the fluid and then perhaps change it again after another 1000km.