Reddit reviews A-Tech 16GB Kit (2x8GB) ECC RDIMM Memory for Mac Pro Mid 2010 & Mid 2012 (MacPro5,1) | DDR3 1333MHz ECC Registered DIMM PC3-10600 Dual Rank 2Rx4 1.5V 240 Pin RAM Upgrade
We found 2 Reddit comments about A-Tech 16GB Kit (2x8GB) ECC RDIMM Memory for Mac Pro Mid 2010 & Mid 2012 (MacPro5,1) | DDR3 1333MHz ECC Registered DIMM PC3-10600 Dual Rank 2Rx4 1.5V 240 Pin RAM Upgrade. Here are the top ones, ranked by their Reddit score.
Designed for the Apple Mac Pro Mid 2010 & Mid 2012 - (4-Core, 6-Core, 8-Core, 12-Core) | Model Identifier: MacPro5,1 | Part Numbers: MD771LL/A, MC561LL/A, Build to Order ModelsCapacity: 16GB | Quantity: 2x 8GB Modules | Speed: 1333MHz PC3-10600 (PC3-10600R) | ECC Type: ECC Registered (ECC RDIMM) | Rank: 2Rx4 (Dual Rank x4) | Voltage: 1.5V | Form Factor: 240-Pin DIMMCompatible with Intel Xeon Cpu Processors: 4-Core 2.4GHz E5620 | 4-Core 2.8GHz W3530 | 4-Core 3.2GHz W3565 | 6-Core 2.4GHz E5645 | 6-Core 2.66GHz X5650 | 6-Core 2.93GHz X5670 | 6-Core 3.06GHz X5675 | 6-Core 3.33GHz W3680 | NOTE: Mac Pro systems with E5620, W3530, W3565, or W3680 CPU Processors will run memory at 1066MHZ (PC3-8500)NOTE 1: This item is ECC memory and is NOT compatible with MacBooks, MacBook Pros, iMacs or Mac Minis which only support NON-ECC SODIMM Memory. | NOTE 2: The Mac Pro Mid 2010 & Mid 2012 (MacPro5,1) supports both ECC UDIMMs (ECC Unbuffered DIMMs) and ECC RDIMMs (ECC Registered DIMMs), but these two types of memory cannot be mixed. Mixing ECC UDIMMs and ECC RDIMMs will result in a memory error when you start up your Mac Pro.Backed by A-Tech's Lifetime Warranty | United States Based Technical Support
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)
I would need a 16 GB DIMM. OWC actually seems to be quite price competitive on those at $94.79. I actually prefer them to Amazon, because Amazon shipping options do not seem to be compatible with my apartment complex. Altho this $60 stick might tempt me:
https://www.amazon.com/Tech-PC3-10600-1333Mhz-MacPro5-MD771LL/dp/B01C7YYR4Q/ref=sr_1_4?ie=UTF8&qid=1524435846&sr=8-4&keywords=1333+mhz+ddr3+memory+module+16gb&refinements=p_n_feature_five_browse-bin%3A677429011
I'll have to call OWC on Monday and see what they have to say.