Sorry for all the confusion. I’m not explaining this as well as I could be. You should purchase 1333MHz ECC RAM, the OWC may work but you shouldn’t risk it. If you are looking to add 2 more sticks of 8GB each for 16GB more, here’s the link to the correct kits.

OWC on Amazon $90: (https://www.amazon.com/gp/aw/d/B01K22UHSK/ref=mp_s_a_1_7?ie=UTF8&qid=1525652826&sr=8-7&pi=AC_SX236_SY340_QL65&keywords=1333+mhz+ddr3+mac+pro&dpPl=1&dpID=41ng6tr-Q-L&ref=plSrch)

And cheaper A-Tech from Amazon $60: (https://www.amazon.com/gp/aw/d/B01C7YYR4Q/ref=mp_s_a_1_14?ie=UTF8&qid=1525652826&sr=8-14&pi=AC_SX236_SY340_QL65&keywords=1333+mhz+ddr3+mac+pro)