I am using 64GB of Corsair Vengeance DDR3 1600. It only uses 1.5 volts. This RAM is on the ASUS Qualified Vendor List (QVL), although it has only been officially tested/qualified at 1300MHz). This list is available from the Z9PE -D8 WS product page on the ASUS website Google "asus z9pe-d8 ws". Click on the middle-of-the-page gray tab called "Memory/Devce Support"., then download the three documents found there.
I originally purchased two 32GB kits (CMZ32GX3M4X1600C10) and filled up all 8 slots. Mixing kits like this is definitely not advisable these days with the speeds that they are pushing RAM to. If you need 32GB, then purchase a single 32GB kit. Ultimately I was very fortunate because 1) I got faulty RAM from Corsair (not common), and 2) my bad RAM sticks were found in BOTH of the kits! That meant that Corsair was obligated to replace both kits entirely. And awesomely enough, they were willing to replace them with one 64GB kit! That 64GB kit works flawlessly BTW.... So I'd recommend Corsair and their Vengeance RAM.
Now lots of RAM sticks these days come with cooling fins that could easily interfere with CPU coolers. My Vengeance RAM is that way. But Corsair also makes a low profile versions. Make sure that your RAM is low profile, or that your cooler is high-rise, or the cooler doesn't extend out over the RAM slots.
If Mom is too slow repairing the hole in your jeans then, "sverte ræva og gå naken"