When I first got my Healey running the trans and od was empty. At the time it was hard to find much info on what to use. So I filled it with 90wt gear oil. For the next 20 years it seemed to work fine. Then I read horror stories about using the wrong oil in gearbox. It sat for several years not in use and the od quit engaging. So last fall I finally drained both and filled with 20W50 oil, same as in engine. I was surprised to find the minor drips from same didnt increase. but the od does shift smoother, without delay. I guess when you dont know any better, no damage will occur before your correct the problem.