Optiplex 990 MT RAM compatibility

Dear All,

I have an Optiplex 990 MT PC. I bought it used with 8GB DDR3 RAM, with the following configuration:

DIMM3: Kingston HyperX FURY 4GB 240-Pin DDR3 1600MHz (PC3 12800) HX316C10F/4 CL10 1.5V

 (https://www.newegg.com/hyperx-4gb-240-pin-ddr3-sdram/p/N82E16820104441?Description=HX316C10F%2f4&cm_re=HX316C10F%2f4-_-20-104-441-_-Product)

DIMM1 and DIMM2: Samsung 2GB 240-Pin DDR3 1333Mhz (PC3-10600) 1RX8 PC3-10600U-09-11-A1 CL9 1.5V

(https://www.ebay.com/itm/Samsung-2GB-1RX8-PC3-10600U-09-11-A1-desktop-memory-M378B5773DHO-CH9-/253496596463)

DIMM4: empty

At the time I bought the PC I had no idea, but from what I've gathered recently, this RAM configuration is not appropriate by any means. Still, I have not encountered any memory errors - at least I can't recall any.

My first question, is this current configuration so bad that it must be changed right now? I mean, it seemed to work for the last 3 years or so. What would you recommend?

Also, I am pondering an upgrade to 16GB RAM. According to the user guide, the only way to this is installing 4 modules of 4GB. In this case, the 2GB modules will obviously have to be removed.

Would it be safe enough to buy an identical "pair" for the existing 4GB Kingston module (as much as it's possible), plus a 2x4GB kit of the same type of RAM and pair them all correctly? Or should I sell all I've got, then buy 2 kits of 2x4GB?

When it comes to buying new RAM, would these work?

https://www.newegg.com/hyperx-8gb-240-pin-ddr3-sdram/p/N82E16820104446

Like I said, I'm already using one of this type. However, these are 1600Mhz 1.5V ones, while the official Dell tech guide mentions 1333Mhz RAM as compatible. On contrary, the Crucial site recommends 1600Mhz 1.35V RAM modules. I'm not sure what's correct.

https://www.crucial.com/compatible-upgrade-for/dell/optiplex-990-mini-tower

Could someone help me sort this out?