Mine did it into 2nd gear. I replaced the fluid with 10w30 motor oil per my owners manual,.and it pretty much went away.

Others swear by MTF

I think the problem is worn synchro gears

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT