MacPro Mid-2010 RAM Upgrade

1333 MHz RAM will work fine provided the manufacturer says they will work in a 2012 Mac Pro.

The only problem you may have is mixing different memory speeds.

Also you can mix ECC and non-ECC memory

Also, you can't mix registered and non-registered RAM. Typically 16 GB and larger sticks are registered.

I upgraded the memory I my 2013 Mac Pro (1866 MHz) and the removed memory worked fine in my 2009 Mac Pro but ran at 1066 MHz

Apparently yes, but you may wish to call OWC as this info came off their site for the mid 2010 mac pro

1333MHz & 1066MHz ECC Mac Pro Memory Information

• Up to 128GB of Total Memory
• 4 or 8 Total Slots
• Apple Specified Thermal Sensor
• DDR3 PC3-10600 ECC 1333MHZ or PC-8500 ECC 1066MHz
• 72 Bit (64 bit + 8 bit ECC)
• High-Performance Multi-Channel

https://eshop.macsales.com/shop/memory/Mac-Pro-Memory?_ga=1.237071296.1701889142 .1465419476#1333-memory