After driving a 1900 CSS for more than 25 years, I’d like to give a helpful tip to other 1900 owners, especially for the gearboxes. 
When buying my car in 1987 the pre-owner gave me several tips on how to use the car without getting problems immediately.

The most necessary item for the gearbox, he mentioned, is to drive it with non-alloyed gearoil only, since the materials inside the gearbox, such as cogwheels and bearings, are mostly out of very soft material and modern alloyed gearoil is too aggressive for this material. Nowadays you’ll find this gearoil by visiting garages of trucks and farmer machines.
[Ed. note: in USA, years ago, the usual recommendation for my 2600 was Shell Dentax 90, not the more modern Dentax 90EP. I'm guessing this also works for 1900 gearboxes as well.]

During the last few years, I saw a lot of broken cogwheels in 1900 gearboxes and, every time, users had used the modern 80/90 alloyed gearoil. From time to time, when I change the gearoil in my car, there is always a brass-coloured cover shining on the surface of the old gearoil-- indicating that material of these times were softer than several years later.

Feel free to contact me if You have some more questions concerning the 1900 gearbox: I have several 5-Speed-gearbox parts as well as complete gearboxes.

--Bernd Kussmaul, Germany

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