Most of the time they are compatible over 99%. Memory has no issues running slower than their rated speed.
ASRock H61M-DGS LGA 1155 Intel H61 Micro ATX Intel Motherboard
Supports: DDR3 1600/1333/1066 DDR3 1600 with Intel 22nm CPU, DDR3 1333 with Intel Sandy Bridge CPU.
If it supports DDR3 1333, can I choose any 4gb, 8gb that's not in the supported list?