More Ram with 8GB Kingston HyperX Genesis Grey

I have the following m/b & ram, Asus P8Z68-V / 8GB Kingston HyperX Genesis Grey (2x4gb), I would like to get another couple of sticks to put my ram up to 16gb.
Will I need to get another 2x4gb exactly the same or can some different types be mixed
Thanks
Looks like my question was answered in a post after mine

I'm afraid no definitive answer, my friend. Some motherboards don't mind mixed Ram whereas some are very picky. No way to be sure but try. Example: I used to use 1x2gb Hynix 800mhz stick and 3x1gb 667mhz Samsung sticks all running at 720mhz in dual symmetrical mode on an old P5Q Asus board. Cpu/Dram ratio used to be important, but not so much anymore now that it has been de-coupled.

I used a set of 2x4GB hyperx genesis grey modules together with a pair of older 2x2GB corsair dominator modules with no issues. (in an old amd phenom system)

When I upgraded to X79 and quad channel ram, I bought a new set of 2x4GB hyperx blue modules to match the older grey set. Been working fine for 2 years now, but I had some slight teething issues initially. tRFC had changed between the old grey modules and the newer blue ones, from 128 to 256. (in clocks, which I think translates to 260ns, the old genesis grey are 160ns)

The old grey genesis modules are from 2011 and are xmp 1.2 and report as 1333 in cpu-z even though they are 1600. The blue ones are from 2014 and are xmp 1.3 and rated as 1600. They both have the same part numbers and 9-9-9-27 timings at 1600 1.65V.

Sorry to piggy back, I have exactly the same problem with exactly the same RAM but on a Asrock Extreme4 Gen 3 motherboard.

I have 2x4GB old grey hyperx genesis, if I get hold of 2x4GB second hand newer blue genesis what kind of problems am I likely to get and what sort of thing might I have to do to get of the 'teething issues' Kei talked about?

In theory the motherboard should automatically run all memory at the speed of the slowest ram, in reality though you may find you have to manually set the timings to the slowest ram.

Personally never had an issue with mixing and matching different memory.

In my old rig I have a P8Z68 V Pro and that has 2 x 8gb sticks and 2 x 4 gb sticks and runs fine so you will likely be ok. My sticks are all the same model (Corsair Vengeance ddr3 1600)