I made this short guide about SODIMM Synology memory. I didn’t test those models, but after a lot of searching and reading, I think they should work. And there are more models from different brands that should work. Feel free to add more recomendations.
<<< Synology SODIMM memory replacements guide >>>
Some older models of Diskstation don’t support add-on memory. The one included is soldered onto the motherboard and can’t be upgraded. Some newer models have 1 dimm soldered onto mobo and 1 slot for add-on dimm. Others come with 2 or more memory slots, one occupied and others free; you can install dimms in all slots, replacing the preinstalled one. You can also use 8 and 16GB in 2+4GB models. Intel CPU supported memory claims are misleading in many cases.
You must use memory with the same specs as original Synology one, including Ranking.
You will find Single Ranking mentioned as 1R or SR, and Dual Ranking as 2R or DR. Different ranking might work, but is better to stay on the safe side. There is no logic relation between Synology name and Ranking, because they use S01 and S02 for 1R and 2R differently, for different capacities, just to annoy customers.
Speed can be mentioned by different manufacturers in 2 ways, like: PC4-21300 or PC4-2666.
There are 3 manufacturers of memory chips: Samsung (the best), Micron (second best) and SK Hynix (third option). Some chinese manufacturers also poped up recently, but… let’s use what we are used too 
. This top is based on performance and tuning/overclocking capabilities of the VRAM in video cards, especialy for mining purposes. For NAS RAM, the differences between the 3 manufacturers, I think, are insignificant, because no tuning is involved. As a special mention, Synology dosen’t make memory dimms. The OEM ones are just rebranded from other manufacturers, and they use chips from the above 3, and I’m pretty sure they choose the cheapest one (aka Hynix).
If you can’t find details about ranking, you can identify it visualy. The ECC version has 1 more chip per rank. Here are all the configs for DDR4.
Number of chips per PCB:
none-ECC unbuffered: number of chips = (data bus/chip bus)*ranks
ECC unbuffered: number of chips = [(data bus/chip bus)+1]*ranks
none-ECC unbuffered: 64 bit data bus
1Rx8 - 8 chips
1Rx16 - 4 chips
2Rx8 - 16 chips
2Rx16 - 8 chips
ECC unbuffered: 64+8 bit data bus
1Rx8 - 9 chips
2Rx8 - 18 chips
Here are the Specs for OEM Synology SODIMM DDR4 and some replacements.
I included Arch memory just as refference, for specs and pictures, but I don’t endorse them in any way.
I also included Kingston because they show all the specs and is one of the top brands.
Crucial dosen’t offer ECC SODIMMS.
D4NESO-2666-4G > 4GB DDR4 non-ECC Unbuff. SODIMM 2666MHz PC4-21300 260pin 1.2V CL19 1Rx8
- Arch Memory SY4GB2666SOr1b8-TZSpecific 4GB SR 1Rx8
- Arch Memory SY4GB2666SOr1b16-TZSpecific 4GB SR 1Rx16 -just for DS220+
- Kingston KVR26S19S6/4 4GB 1Rx16 -just for DS220+
Must use 2R over 8GB. These should work too:
- Kingston KVR26S19S8/8 8GB 1Rx8
- Kingston KVR26S19S6/8 8GB 1Rx16 -just for DS220+
- Kingston KVR26S19D8/16 16GB 2Rx8
D4ES01-4G > 4GB DDR4 ECC Unbuffered SODIMM 2666MHz PC4-21300 260pin 1.2V CL19 1Rx8
-Arch Memory SY4GB2666SOECr1b8-TZSpecific 4GB 1Rx8
D4ES02-4G > 4GB DDR4 ECC Unbuffered SODIMM 2666MHz PC4-21300 260pin 1.2V CL19 2Rx8
-Arch Memory SY4GB2666SOECr1b8-TZSpecific 4GB 1Rx8
D4ES01-8G > 8GB DDR4 ECC Unbuffered SODIMM 2666MHz PC4-21300 260pin 1.2V CL19 2Rx8
-Arch Memory SY8GB2666SOECr1b8-TZ01 8GB 1Rx8
D4ES02-8G > 8GB DDR4 ECC Unbuffered SODIMM 2666MHz PC4-21300 260pin 1.2V CL19 1Rx8
-Arch Memory SY8GB2666SOECr1b8-TZ01 8GB 1Rx8
-Kingston KSM26SES8/8HD 8GB 1Rx8 Hynix
-Kingston KSM26SES8/8MR 8GB 1Rx8 Micron
D4ECSO-2666-16G > 16GB DDR4 ECC Unbuff. SODIMM 2666MHz PC4-21300 260Pin 1.2V CL19 2Rx8
-Arch Memory SY16GB2666SOECr2b8-t001 16GB 2Rx8
-Kingston KSM26SED8/16HD 16GB 2Rx8 Hynix
-Kingston KSM26SED8/16MR 16GB 2Rx8 Micron
D4ES01-16G > 16GB DDR4 ECC Unbuffered SODIMM 2666MHz PC4-21300 260pin 1.2V CL19 1Rx8
-Arch Memory SY16GB2666SOECr1b8-MGSpecific 16GB 1Rx8
-Kingston KSM26SES8/16HC 16GB 1Rx8 Hynix
-Kingston KSM26SES8/16MF 16GB 1Rx8 Micron