For the models in the knowledge base, the maximum RAM is:
- RS1219+: 16 GB total, 2 slots, 8 GB x 2 [1]
- RS2818RP+: 64 GB total, 4 slots, 16 GB x 4 [3]
- RS2418+: 64 GB total, 4 slots, 16 GB x 4 [5]
- SA3600: 128 GB total, 4 slots, 32 GB x 4 [2]
About third-party RAM: the knowledge base says Synology does not provide complete product warranty or technical support if non-Synology memory modules are used for memory expansion, and that Synology memory modules are recommended for optimum compatibility and reliability [1][2][3][5]. It also notes that third-party RAM may be reported by users as compatible, but stability and compatibility cannot be guaranteed [12][14].
So the safe answer from the sources is: use Synology RAM if you want the best compatibility and support; third-party RAM may work, but it is not officially guaranteed [1][2][3][5][12][14].
Sources used: [1], [2], [3], [5], [12], [14]