Remember that second hand HDD's tend to fail around the same time (when they are the same brand, and have had the same use-time). So if your data is precious, don't forget to also backup your data on the NAS. Also check which HDD's they are. there are harddisks for consumers (where the hdd is not a long running thing, and HDD's for servers). The ones for servers are slower, but tend to stay better when running continuously (something to do with the oil on the platter and centrifugal forces)just picked up a Synology DS418 NAS with 3 x 8TB HDD in it for $330 Aussie dollars and a GTX1070 for $250 Aussie dollars...
I always was a bit hesitant to buy WD, mostly due to the pile of failed drives I pulled out of systems in the early 2000s, just gave me the feeling of cheap and unreliable.I've had three WD blue die on me after very short time. Never again. The reds have been great so far.
There should be nothing on the platters. If there were a liquid on there the floating head likely would get dragged down and crash into the platter. In my experience the only discs that spin slower are those that are defined for closed circuit TV applications.
See the lubrication layer. PFPE falls under the Synthetic oils .
edit: Removed the part about chaff (which also occurs) because I wanted to keep it on the topic of the outerlayer of a harddisk only.
You can also use motor oil, as explained here: https://groups.google.com/g/comp.sys.ibm.pc.misc/c/v2ktyDp4Ixw
Should also work for SSD