worn synchro

I have a 1979 240 D with 227,000 miles. When either down or up-shifting into 2nd gear there is a grinding sound unless I shift very slowly or double clutch. I am told this is a worn synchro on the 2nd gear. The interesting thing is that this grinding only occurs when the engine is warm. When the car is first started, I can drive several miles shifting normally. I am guessing that when the gearbox oil is cold, the viscosity is thicker and this somehow aids smooth shifting. My question is this: can I replace the gearbox oil with a thicker viscosity so that even when warm, the oil will be thick enough to allow a smooth shift? Any ideas?