If upgrading the Synology 1019+ from 8GB to 16GB, can I use either of the Crucial (8Gbx2) 1600 MT/s or 1866 MT/s version?
Also, would the same Crucial (8Gbx2) kit work for the DS918+?
DS1019+ just like DS918+ feature exactly the same CPU and therefore supports exactly the same RAM. I would suggest Crucial 8GB Single DDR3 1866 MT/s (PC3-14900) 204-Pin SODIMM RAM as it is tested and proved working. And it is also faster then 1600 MT/s model. But if you find slower RAM cheaper, it should work. Just do not mix different memory sticks.
Crucial 8GB Single DDR3 1866 MT/s (PC3-14900) 204-Pin SODIMM RAM