Re: Can I safely add the two Crucial memory modules to my 9020 with the existing Kingston modules, even though they have different voltages? DDR3L is a dual voltage capable memory SoDIMM, which supports operation at both 1.5V and 1.35V.  DDR3L is also pin-compatible with DDR3. DDR3 is a single voltage capable memory SoDIMM, which supports 1.5V operation only. your 9020 memory slots support 1.5V only.  You should be fine running the two ddr3L and the two ddr3 together.  all of them will be running at 1.5V. install the two 8gb in same color slots, and the two 4gb in the other two same color slots, whichever order you prefer.
I recently purchased a refurbished Dell OptiPlex 9020 MT, Intel Core i7-4770 3.40GHz, 8GB memory, 1TB HDD running Windows 10. The memory currently installed are two 4GB Kingston memory modules (PC3 - 12800U, product # K531R8-HYA) running at 1.5 volts. The modules are in slots #1 and #2 among the four slots available in the 9020 MT. I have on-hand, two 8GB Crucial 240-Pin DDR3 SDRAM DDR3L 1600 PC3L 12800 (model CT102464BD160B) memory modules rated at 1.35 volts. Can I safely add the two Crucial memory modules to my 9020 with the existing Kingston modules, even though they have different voltages? If so, does it matter which pair of modules go into which slot…e.g. put Crucial modules in slots 1 & 2 (since they're newer and larger), and Kingston modules in slots 3 & 4? I assume it should be OK since looking at the Crucial memory documentation it lists voltages of 1.5V and 1.35V with a footnote: "DDR3 UDIMMs are 1.5V only. DDR3L 1.35V UDIMMs are also 1.5V capable." I just wanted to make sure it would not cause physical damage or slow down memory access...and if there was a preference of which slots the modules should be installed into. Thank you.
OptiPlex 9020 MT memory upgrade.