I have always resisted the "filled for life" approach, whether this is brake fluid, coolant, gearbox oil or axle oil. "Things" wear. That;s what "Things" do.
Brake fluid comes out dark after two years. Gearbox oil and diff oil comes out dark after a couple of years, even when "they say" it is good "for life".
So my approach has always been to replace such lubricants as I feel necessary, and never regretted that, after seeing what comes out. I also accept the alternative approach to fit and forget, but that's not the way I do it.
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