Hi Adrian

1- file with all decoder types: No and since the huge production of Märklin locos I have a doubt some has it EVEN one model number may have multiple decoder types over the years
C80, C90, Systems, mSD, mSD3. Usually a picture of the decoder will allow forum members here to help identify the type.

2- Serial number. You may want to read this post describing the serial number structure on M. locos
https://www.marklin-users.net/fo...comotive-s-serial-number

3- What oil do you use for maintenance, to unclog the wheels.. For locos past 2000, the easiest is to go to Märklin website (Germany) and type your model number in the search field
You may find the user's manual, the parts (exploded) view diagram

Unclog the wheel: It is often about a foreign particle (ballast piece) in the geartrain. There are videos on you tube type Märklin + model number + maintenance in the search field
Usually, a bath in RS24 (Noch with a brush ONLY of the mechanical parts will solve the problem

Your picture shows a "Quartering" issue. On recent steamers, only one axle is driven by the motor and the other axles are using coupling rods. The wheels must be QUARTRED (at a 90° angle between one side and the other. The first time it happens, no big deal you may rotate the wheel back in its place while paying attention to the quartering and to the wheel spacing (13.9 mm)

4- Black rubber on some wheels: they are called traction tires and increase geatly the locos pulling power. When no damaged you may clean them using RS24. Replacement on Märklin shop or at you Märklin dealer is avalable (5 different types I think)

5- Using locos on an old transformer: YES there is danger to "fry" a modern decoder when using old transfos. This is because the invertion overvoltage may be WAY BIGGER TAHN the 24 VDC

Cheers
Jean