I have a '14 Impreza w/ 30K miles. Whenever I get the oil changed they're always trying to sell me front and rear differential flush service and saying it's recommended by Subaru.
I'm a home shop machinist, so mechanical things and metal properties make sense to me. I intuitively think that the diff fluid wouldn't reach a temperature high enough to cause oil breakdown nor should there be enough gear wear to cause particulate sufficient to cause any problems with the thing.
Is such a service really necessary or are they just trying to get a quick buck? What really is going to happen to the diffs if they don't get flushed?
James2016-03-17T13:19:12Z
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LOL. A differential cannot be "flushed". It's totally a scam. They are absolutely lying to you.
HOWEVER -- it is critical to replace differential fluid at the manufacturer's recommended intervals, which can be anywhere from 30,000 to 100,000 miles. It's not "flushed", simply drained out and replaced with fresh oil.
Why? You need to think better about this. The gears churning through the fluid aerate the oil, and it thus is exposed to loads of oxygen, so it oxidizes. Furthermore, the intense pressure of the gears can reach thousands of PSI where they touch, and this shears the oil molecules down into smaller chains that are less effective at lubrication. Furthermore, the friction-modifying additives in the oil that minimize gear and bearing wear do also break down over time.
Not only that, but differential oil certainly *does* reach high enough temperatures to break down. That's why high performance vehicles have differential coolers. Even Camaros since 2010 have this as an option, and it is required on the most powerful model. Last, if you'd ever had a differential apart, you'd know there are often *lots* of metal particles in the oil from wear from the gears running in. Most of this occurs in the first 15,000 miles, so that first service is *critical* to minimize future wear. If you have a clutch-type limited-slip differential, there are wear particulates from the clutches as well - not as damaging as metal pieces of course, but undesirable. Since differential oil is not filtered, all that metal and gunk circulates around in the differential, causing further damage.
Don't follow what repair shop says but follow what your maintenance schedule in your owners manual says about how often the differential fluid should be changed.