Birch oil and tar is distilled from Birch bark.
Making Oil From Birch Bark
from 07.08.2013
from
Far North Bushcraft And Survival
is fairly good, to thicken the oil it can be boiled, just take care it doesn't catch fire as the lighter fractions evaporate.
I felt sad to see such thick birch bark being used for the process, the climate is mild where I live so finding bark of reasonable thickness is very difficult.
I treasure any pieces I find over 3mm thick with a good surface.