My truck has a 4000# rear axle load on it 24/7 due to the camper which isn't all that much. I installed a Mag Hytec@ 9200 miles and Amsoil 75-90 Severe Gear. The stock fluid was jet black and smelled awful. In my case it was worth it.
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'07 2500 SLT,5.9,reg.cab.,G-56, 4wd,8' Northstar popup 24/7. Carli,Defiant,Kore,Lorenz and MADS.