Maru evaporates very quickly, so lubing the core with it wouldn't help much in the long run. If you don't want to use lubix you can get some diff oil instead.
got them from speedcubeshop, asked them what they lubed it with (because I also thought they come without lube) and they told me they didn't lube it, it came like that from the factory...
What individual step times do you need for averaging around sub-15?
Like, cross+F2L = 9
OLL=3
PLL=3
etc.
Also, how much practice would it take from sub-20 to sub-15?
all of them.
the letters are just different versions / added steps to complete the cube in a slightly different way.
Use the guide linked on the ZZ wiki page.