My NP200 has been thoroughly cleaned, overhauled with new bearings and seals, and correct preload, and it still makes noise at anything over 40 MPH and gets hot at any extended running over 50 MPH.
It takes about 30-40 minutes of steady highway speed and it will get up to around 190 degrees (read on a temp guage installed in my transfer case) on a warm day. Driving further on for another 20 minutes or so it will climb to 220 degrees. After that, it doesn't take long for it to shoot up to the end of the scale if you keep pushing it.
Running synthetic or conventional gear oil makes no difference in temperature either. They are just an antiquated design, plainly put.
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