I don't know enough about the process to know how necessary it is nor what the pressure involved is. But the machine shown in the photograph in most of the MOLO's operates at what I would guess is a fairly high pressure.
As far as material to use to print gears. PLA the easiest to print is the most brittle and weakest, however oil doesn't degrade it. ABS I would NOT recommend because of the problems with both warping and need to exhaust the fumes, PETG is better here, but Nylon is even better.
IF you want to learn a bit more four people with u-tube channels I could recommend are;
Thomas Sanladerer:
Makers Muse
3D Maker Noob
3D Printing Nerd
One last thing, I am waiting for the Prusa MK3 from Prusa Research, because I want a better printer to work with, and Josef Prusa is the originator of the I3 design, the chinese are so fond of cloning. Should arrive at the end of the month.
The prices I posted are replacement metal parts which should be the equal of Zamak originals. Use those prices as comparison when deciding if printed plastic gears are really worth buying. They are NOT 3D printed plastic! I do not know if they are new old stock or modern manufacture, but they are from the company that bought Atlas decades ago.