Not many people realized this, but Synology has their own brand of RAM.
Officially, Synology always recommends you to use their brand of RAM for 100% compatibility and reliability. It comes with three years warranty and is made in Taiwan.
For the ECC module, there is only one variant which is RAMEC1600DDR3, and it is available in 2x 2GB, 2x 4GB or 2x 8GB configurations.
For SO-DIMM, there are three variants, D3NS1866L, RAM1600DDR3, and RAM1600DDR3L.
Both the D3NS1866L-4G and RAM1600DDR3-4G are available in 1x 4GB and hence the -4G at the back of the model number.
The RAM1600DDR3L is available in 2x 4GB or 2x 8GB, and hence the model number is RAM1600DDR3L-4GBx2 and RAM1600DDR3L-8GBx2 respectively.
The D3NS1866L-4G is a 4GB DDR3L-1866 unbuffered 204-pin SO-DIMM running at 1.35V. The RAM itself is manufactured by SK hynix.
The upgrade will expand the RAM of my DS918+ from 4GB to 8GB and DS418play from 2GB to 6GB.
You can get Synology RAM from Storage Studio starting from next month, December 2018. I am still waiting for the local price, but the official RRP for the D3NS1866L-4G is US$70 (S$96).