Non-detergent mineral oil was originally specified in the manuals. The theory, at least what I've heard, is that the tiny particles of ground metal will settle to the bottom of the case forming the 'mud,' whereas a modern detergent/dispersant oil would cause the particles to be suspended, as Michael attests. When I do change my gearbox oil, there will be tiny particles--judging by the color they are brass, likely from the shift forks wearing--that are visible in bright sunlight (kinda pretty actually). I was alarmed when I first saw this, but rubbing the oil between fingers I could tell the particles were extremely small, so small you could not feel them at all (note some greases; moly, lithium, etc. consist of finely ground metal thickening an oil). My take is the suspended particles are so small they do no damage--brass is a fairly soft metal--and will not cause any passages to be blocked. I don't know exactly how long I've been running synthetic gear oil in both my boxes, but it would be at least 75K miles, and I've had no problems that could be attributed to using synthetic. As a bonus, a good synthetic gear oil will give noticeably smoother shifts and, since the viscosity is more consistent an OD will engage slightly faster (esp. when hot). There is never any mud when using a proper synthetic oil; note I don't use synthetic oil in my engines, as I change the oil often relative to my daily drivers, and the synthetic will leak a bit more than conventional gear oils. I recently rebuilt my BJ8's box, and paid particular attention to surfaces and gaskets, and used a good sealant on the paper gaskets, and there are no significant leaks once I solved the activating cross shaft O-ring leaks.