MacPro 6.1 memory in 5.1

Would testing 2013 MP 4 GB sticks in a 2009 Mac provide and such test?

I upgraded my 2013 and still have the original 4 GB sticks. I do not want to damage either the memory or the 2009 MP

I just tried the original memory (4x4GB) from my 2003 Mac Pro (it now has 4x8 GB) in my 2009 Mac Pro and it seems to work fine; not problems yet. It is identified as 1066 memory

I think I meant to write:

"Faster" Mac Pro late 2013 memory

... above, but got spell-corrected into something else

Micron pc3-14900e-13-13-d1 1866MHz 4GB

The RAM from the late 2103 Mac Pro would be DDR3 ECC 1.5 Volt, running at 1866 MHz

The RAM from the Mac Pro 2012 would be DDR3 ECC 1.5 Volt running at 1333MHz

The RAM from the Mac Pro 2009 would be DDR3 ECC 1.5 Volt running at 1066MHz

>> IF and only if the RAM could step down correctly to run at slower speeds, you could use Mac Pro 2103 RAM in an earlier Mac Pro (2009 through 2012).

It is a compatible type, DDR3, and the key on the RAM slot would accept it, and it would do no damage (provided you did not use a Hammer during installation) but it may or may not work. The manufacturer must have taken steps to assure that the faster RAM would also run at slower speeds with the appropriate slower timing.

None of this would work in older 2006-2008 models that require matched pairs FBDIMMs, a different type with a different key in the memory slot.