I'm going to go against 'common wisdom' here on the 'non-detergent lets the crud sink to the bottom, which is a great thing' theory*. The last couple times I changed-out gearbox oil in my BJ8 I drained it into a clean pan, and looked at it in bright sunlight. The oil did have an extremely fine brass 'shake,' that was only visible in the bright sunlight; it looked like a really fine metal-flake paint job (kinda pretty, actually). I rubbed some of this oil between my fingers and could feel absolutely ... well, nothing but oil. No grit or anything potentially abrasive. The brass particles--likely from the shift forks wearing down--were so tiny I can't imagine them plugging any passages in the gearbox or OD. Brass is a soft metal, and would be ground to virtually nothing in a gearbox; note many greases are made with finely-ground metal (lithium, moly, etc.). When I rebuilt the box a couple years ago the inside of the box was spotless, but there was a few small grains of silver metal on the magnet in the OD strainer. I've never had any OD issues--that weren't electrical--whatsoever (I rebuilt the box because is was jumping out of fourth). The gearbox had no appreciable wear at 200K miles, but I replaced the fourth gear cluster to remedy the jumping-out issue.
Anybody ever have a failure of a gearbox or OD that can be definitively attributed to using detergent or synthetic oil? If not, I'm putting this one in the "old wives tale" category.
* Note 'detergents' were added to oil to do what detergents do; i.e. clean things out. In engines, it's used to clean lead--when autogas was still lousy with the toxin--and other deposits in the engine; I believe ZDDP--'P' is for phosphate, which is a key ingredient in many detergents--was first added as a detergent, then the anti-wear properties were discovered. 'Dispersants' are what's added to oil--as the name applies--to keep particles in suspension. A non-detergent oil will not necessary be non-dispersant, but I do think they usually are.