And I have mixed operation with the 4GB from the basic equipment with my 12GB. So far without any problems.

Hello Terapix29,
As far as I know, there are several possibilities:
1. the original storage from Synology. They are expensive. Synology has the RAM manufactured by memory manufacturers selected by them and tests their devices extensively with it.
2. search yourself at the memory manufacturers, e.g. Kingston, etc. You have to know your way around and expect that attempts will fail. I.e. then exchange etc. It takes time and sometimes costs nerves.
There are also suppliers who offer memory expansions to match the device, e.g. ds1821+. E.G. https://www.speicher.de. This is not original memory, but from well-known memory manufacturers such as Samsung.
Important for 2. and 3.: If there are problems, Synology understandably only accepts devices with their original memory.
Also important: The ds1821+ needs ECC memory, i.e. memory with error-correcting code. And the more memory the better. Maximum 32 GB, i.e. 2*16 GB ECC.
Good luck.

Hier die offizielle Kompatibilitätsliste von Synology
https://www.synology.com/de-de...