I can't imagine 40 weight oil would do the job... maybe, I never tried it. I always used, and always found, 90 weight gear oil in the transmissions. The 90 weight lube has a distinct odor. You smell it when you open the case and every OD / trans I ever opened wreaked like gear oil! That's all I've ever used and it's always worked fine. Take that for what it's worth.
