Tbh Genos shouldn’t have black oil tears it should be transparent oil tears
Black oil tears = crude oil tears = low efficiency fuel
Transparent oil tears = higher fraction oil tears = high efficiency fuel
Low efficiency fuel = incomplete combustion unless Genos can crack the crude oil internally
Plus crude oil is toxic for humans and Genos wouldn’t probably want to potentially cause harm to his own brain(if that’s still organic anyway) by having crude oil near it, or to his sensei.
Unless the crude oil is why Genos’s sclera is black, then that makes more sense