I've let my gearbox/OD oil go as long as 20K miles. When I drained it into a clean pan there weren't any solid particles and the oil looked, smelled and felt new. There was an extremely fine brass-colored shake in the oil--kinda pretty in the sunlight--but there was no gritty feel whatsoever. Note some greases are oil thickened with soft metal--lithium, moly, etc.--and brass is a soft metal. When I did rebuild the gearbox there was no excessive wear, just the 'usual,' like the ends of the layshaft.