There are 2 kinds of corksheets.
One is a thick one, the one used in head gaskets etc.
The other one is a thin one. just like a thin cardboard.
Which one should I get and from where?
A better solution could be to try a thick o-ring around the cap.
No ring available.
Only the new oil cap is available and its plastic quality is pathetic.
Also the oil that was leaking initially, was it engine or gear oil?
Both were leaking.
The oil chamber seals and the gearbox seals both were damaged.
We got both of them changed and that solved the problem.