I added another 32GB to my 923+ recently. Looking around, I found that speicher.de (German shop specializing in RAM) offers RAM for specific Diskstation models beyond Synology's specification, and they guarantee you that it's going to be working! (They call this "overRAMing"). For the DS920+, you can find the offered RAM sizes here. I have had zero problems, my NAS now has 36 GB RAM, shows no errors or warnings, and I'll probably upgrade it to overall 64 GB sometime. And my 218+ runs with 10 GB RAM since several years without any issues or warnings. Rather the opposite: in both cases, the difference was tremendous, it felt like entirely new (much faster) devices. I can highly recommend.
[Edit: I changed the German links to the corresponding ones of their English site]