[SOLVED] Is this type of RAM compatible with my Optiplex 390 SFF?

Dells require low density RAM. x64 in the oart # is one way to identify it. X128 "AMD" RAM won't work. UDIMM non ECC low density DDR3 modules.
https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-390-desktop#MEMORYFilters
Forget the 12GB 2 modules. That either mismatched sizes or total BS.
Officially 8GB 2x4GB rating is because 8GB modules didn't exist when that computer was made. It wasn't sold with them. You can run 2x4GB or 2x8GB. There are no 6GB modules. 8GB should work very well unless you have a 4GB or more GPU or some other special need like video editing.