iMac 2012 RAM upgrade with mixed voltages Variation of already asked question: 

I now understand that I can add two matched 8 GB sticks of RAM into the two empty slots, even though the other two slots are filled with two 4 GB modules. But is there a problem having one pair 1.5v if the other pair are 1.3v?

It would appear to not be a problem, if as reported elsewhere, the iMac has 1.5v preinstalled and Crucial is recommending 1.35v sticks. Is that a faulty assumption?
I wouldn't use anything but 1.5v RAM in the 27-inch iMac (Late 2012) as that's what my go-to Apple Memory site lists as compatible.


Ramjet recommends DDR3-1600, 1600Mhz, Non-ECC, Unbuffered, 1.5V, 204pin SO-DIMMs


I also can't believe I'm seeing 32 GB of memory for sale for just under $260 as that just seems too cheap to be true.

If you are looking elsewhere for RAM, I would encourage you to select non-stacked chips on the SO-DIMM as we don't know how warm these devices run and the last generation of 27-inch iMac put out significant heat and had a lot more aluminum to radiate that energy before a fan and convection were needed to assist in heat dissipation.