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What Causes Gear Lube to Darken and Thicken?
 
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Question:
What Causes Gear Lube to Darken and Thicken?
I drained the fluid in my Dana rear axle recently and replaced it with Mobil 1™ Synthetic Gear Lube LS 75W-140. I noticed that the old gear lube was very dark and thick. Do you have any idea why this was? Do manufacturers put special break-in oil in the diff or is it due to normal wear? My 2012 Nissan Xterra has less than 1,500 miles on it.
-- Bruce Slomovitz, Mt. Pleasant, SC
Answer:
The fact that the oil turned dark is very normal. This may be from normal oil break down for heat and degradation due to service. As oil heats up during normal operation, it begins to thicken due to oxidation.
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