Reading 2015_PSDs posts about his mag hytec diff covers and fluid change got me thinking that its time to do it in my Ranger which im almost certain has the factory gear oil. As of this morning when i parked it, it has 138,385 miles, so Im sure that gear oil is pretty nasty. Using the axle code found on my door sticker and a chart I found online, I was able to determine that my truck has a 7.5" open differential and 3.73 gears. According to Amsoil I can use 75W90, 80W90, (which I think is factory recommended) and also 75W110. Being its open, does that mean I dont have to use a limited slip additive or gear oil with additive in it? I have some Amsoil 75W90 that is about 7-8 years old, its in the original cardboard box and the bottles are brand new and sealed but its in a deteriorating barn and gets exposed to moisture, should it be ok to use? Or should i step up to 80W90 or 75W110? Also would it be better to use silicone when I put the cover back on or should I use a gasket?
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