Confusion with memory expansion

[xdstep]

So I'm kinda confused I'm going to be upgrading my ram in my rig currently at 8gb using 2x4GB Kingston HyperX Fury - 1866MHz DDR3 RAM but I can't find the same ram so my question is can I use any ram even if it's faster slower etc?
motherboard Asus B150 PRO GAMING D3 Motherboard

 

[CountMike]

What can I say, it might work but maybe not. Things like that are unpredictable. Your chances get better if all sticks are in MB's compatibility list, run at same voltage range and have same range of Cl values. If it succeeds at all, both may run at the frequency of slower one. Sometimes manually selecting settings in BIOS may be required.

 

[Tradesman1]

Best to get full amount in a single package that you want/need - no guarantees when mixing DRAM even if you find the same exact model of what you have.