The “cars” should == “Linear blocks” which are the metal parts on the rails which hold all the bearings.
If they come off the rails, then they will lose their bearings — repacking them is a pain, so don’t let that happen. There are small plastic blocks which are the shape of the rails which can be used to remove them from the rails for maintenance, but that shouldn’t be necessary, just put a small plastic clamp each end of the rails to keep them from rolling off.
Yes, we recommend Mobil Vactra No. 2:
for the linear rails/blocks on the Z-axis. For the round rails for X/Y use Super lube:
as noted, it’s available from some sewing specialty shops. I bought some from Amazon ages ago according to my purchase history: