Dell Optiplex Micro 5000 & Memory upgrade

Ideally, you would install a matched pair of two x 8 GB DDR4-3200. That has the greatest chance of success.

You could try to add a second 8 GB stick of the same specifications, buying the same speed and timings even if not the same brand. It may work. But, sometimes mismatched RAM brands/models may not play well together, which is why a matched pair is recommended.

Try to buy from a vendor that will allow you to return the RAM if it doesn't work.

Follow the disassembly and reassembly steps in Dell's Service Manual.

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.

OK.  Here are my findings regarding this issue:

The Dells are known to be finicky about memory modules, which is why finding an unknown memory module for it is chancy.  When asked about this topic in Dell's community, long-standing members normally recommend the Crucial memory modules, which in the US, have warranties that they work with Dell.

Kingston also has specific web pages dedicated to Dell's products as well, including the Optiplex Micro 5000.

My Optiplex comes with an 8GB SK Hynix HMAA1GS6CJR6N, which has 16 chips on one side, speced to come from a "c-die".

Hynix' website (https://product.skhynix.com/products/dram/module/sodimm.go) indicates 2 more memory modules (with with C-die, and another D-die) with the same timings, both have 8 chips on both sides of the module.

I ended up buying another memory module from a trusted general computer dealer (with a LT warranty from the dealer) with the module number HMA81GS6AFR8N-VK (with A-Die), which is no longer listed on Hynix' website, but probably was (indicated by google search linking to a non-existent page).  This works perfectly well.

I speculate that probably all the Hynix's modules still listed on the website would work as well.

It's noteworthy that although Dell lists specific memory configurations, some configurations that are not listed would work as well, such as adding 4GB to the existing 8GB, as an IT person on this forum mentioned that's what he did for the company's computers.  In other posts, some people are able to exceed Dell's configuration in term of max memory.  This is of course, all speculations/experimentations.  It's better to live in a country where you can actually return the memory module that doesn't work out.