RAM ranks matter in the trash can Mac Pro.

Yes, they must match. The 2013 Mac Pro is picky about memory ranks. You can’t just throw in any DDR3 ECC sticks.

Apple’s spec says all four slots need the same rank configuration—either all single-rank or all dual-rank. Mixing them can cause boot failures or throttled performance. Also, the Mac Pro uses registered ECC (RDIMM), not unbuffered. So even if the sticks are ECC, they need to be RDIMM, and the speed has to be 1866 MHz.

Check your current sticks for the rank marking (often 1Rx4 or 2Rx4). If you want 16GB, you’ll likely need to buy a matched set of four 4GB sticks (all same rank) or go with two 8GB sticks in the right slots, but the trash can only has four slots—and it dual-channels in pairs. Honestly, the easiest path is to buy a known-compatible kit from a vendor who confirms Mac Pro compatibility. OWC is the safe bet.

Don’t waste time guessing. Get a matched set and move on.