When I engage 4WD on my (normally rear-wheel drive) Toyota Hilux (KUN26R) and give it some hard job to do (like drive the front wheels on a 10cm high sleeper lying on the ground, or just turn the truck around in a mess) the front differential suffers. The oil in it would produce terribly smelling smoke venting out of the oil plugs. The area around the plugs would become oily as if the oil has boiled out of there. I do not do this often, but when I do it happens.
Two years ago when I was buying the truck, the previous owner told me about smell when doing hard 4WD job, so I changed oil in all gears. I put one of the best oils possible in there (Amsoil Severe Gear 75W-90). I cannot recall if that helped initially (it probably did), but just one year later a mechanic whom I asked to perform safety inspection told me that the oil in the front diff was a real dark sticky mess. I took that note but decided to monitor the situation rather than change the oil again immediately.
Now it is apparent that the problem persists. I do not blame the oil as it is top-notch. Has the front diff worn out? Does it need replacing?