It was posted a year or so ago that 40 wt oil is recommended for the #4 speed reducer. I presume you are talking about gear oil rather than motor oil. I have not found a source for 40 wt gear oil, but I can get 75/80 wt. Would that present a problem? Alternatively, would 30/40 wt compressor oil work?
Since this is an oil bath, I don’t see the need for gear oil which is a thick clinging oil. 40 wt motor oil should be fine.