EDIT: RE-DOWNGRADING TO 7.1.1 IS POSSIBLE WITH THIS!
I noticed there was no clear tutorial on how to do this yet so I have decided to make a guide explaining how to install the Oreo update on the Moto Z. This should work on most unlocked models (excluding Verizon). I have tried this on my Moto Z RETUS and it is working fine. I believe you should also need an unlocked bootloader, I haven't tested it on a device with a locked bootloader. Please download all the files from the bottom of the post.
Run the following commands with fastboot to install this version of 7.1.1 (download below) (hold power + volume down to get into bootloader mode):
fastboot flash bootloader bootloader.img fastboot flash modem NON-HLOS.bin fastboot flash fsg fsg.mbn fastboot erase modemst1 fastboot erase modemst2 fastboot flash bluetooth BTFM.bin fastboot flash dsp adspso.bin fastboot flash logo logo.bin fastboot flash boot boot.img fastboot flash recovery recovery.img fastboot flash system system.img_sparsechunk.0 fastboot flash system system.img_sparsechunk.1 fastboot flash system system.img_sparsechunk.2 fastboot flash system system.img_sparsechunk.3 fastboot flash system system.img_sparsechunk.4 fastboot flash system system.img_sparsechunk.5 fastboot flash system system.img_sparsechunk.6 fastboot flash system system.img_sparsechunk.7 fastboot flash system system.img_sparsechunk.8 fastboot flash system system.img_sparsechunk.9 fastboot flash system system.img_sparsechunk.10 fastboot flash system system.img_sparsechunk.11 fastboot flash system system.img_sparsechunk.12 fastboot flash system system.img_sparsechunk.13 fastboot flash system system.img_sparsechunk.14 fastboot flash system system.img_sparsechunk.15 fastboot flash system system.img_sparsechunk.16 fastboot flash system system.img_sparsechunk.17 fastboot flash oem oem.img fastboot erase cache fastboot erase userdata fastboot erase customize fastboot erase clogo fastboot reboot
Now, open a new command prompt window, and rename the "Blur_Version.27.11.58.griffin.retail.en.US.zi p" file to "ota.zip". Run "adb sideload ota.zip", and it should sideload the oreo update.
Note: If you have any issues sideloading the OTA (example issue: * cannot read 'ota.zip' *) then download this and move the adb.exe and dlls to the same folder as the ota zip. This version of ADB should be able to sideload the zip without the "cannot read ota.zip" issue. In terminal, make sure to run "adb.exe sideload ota.zip" instead of just "adb".
Finally, use the volume keys to scroll down to "Wipe cache" and press the power button. Then scroll up to reboot and hit the power button again and you should be good!
7.1.1: 7.1.1 Firmware Download
Flashtools zip: Moto Flashtools ADB / Fastboot Download
Oreo update: Download Blur_Version.27.11.58.griffin.retail.en.US.zip (Oreo)
Want to root Oreo? Follow this guide, should work for the normal Moto Z: