since its a gearbox, you can usually tell if there are issues just by visual inspection...milky color means a water intrusion issue and look for mettallic silt in the oil....a little is normal, a lot means something is wearing bad.....unlike engine oil which changes color due to heat and contamination from soot/gas etc...gear oil stays relatively clean unless it was overheated somehow.....every year I change it, the fluid still looks pretty good....
