I use a mixture of hypoid gear oil and 20/50 in the primary and gearbox, although ATF seems popular in the former. I'm not losing oil from the gearbox (not in any noticeable amounts anyway) and the primary is pretty much the same. I do, however, wonder if there's a slight leak on the primary gasket as I do get small amounts of oil collecting there??
The oil doesn't have a smell and seems dark - which suggests it's engine oil. Incidentally, since I've let the oil level in the tank drop a little and reset the timing, the problem doesn't seem as bad as it did?
Any ideas about how to clean oil from the rear tyre effectively??