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?

It probably won't help to change to a thicker oil. If the synchro is bad replacement or double clutching is about the only solution. Double clutching is cheaper.

Just a note to remind that a worn synchro is a pretty strong indication that the car has been driven by someone that signifcanty moves the shift lever BEFORE the clutch is fully disengaged.

I agree, changing oil will probably change nothing.

Good luck,

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Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Have this fixed ASAP. If you start losing pieces of the synchro dogs and rings, you will destroy your transmission

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE