transmission/Gearbox are words for the same thing.. do you mean the rear end of the car?

Keeping your transmission healthy, pretty straight forward. Keep the fluid clean, changed in a regular fashion and the filter too. Remember on some mercedes you have a drain on the torque converter, a small little plug you have to line up with an access point on the bell housing.

If you mean gearbox as the rear end of the car, dont use transmission fluid in it. It cannot take that abuse. use the rear end fluid/Gear oil for it as recommended.

If you can, post what you mean by gearbox... as for the cost. you are looking at maybe 10-20 bucks DIY or over 100 to have a shop do it. compared to a rebuilt transmission though... it's not too bad... the rear end fluid is simple to to, Generally 2 plugs remove both to drain, then put the bottom plug back in, then refill rear end with the right fluid till it comes out of the top hole. put the top plug in and you are done. Generally anyways. there's always different setups to this too

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed