So I went and bought some Yamalube from my dealer for my oil change. She advised me that I should always do my diffs at the same time. I have 800KM (roughly 500 miles) on my grizzly. The manual says it should be done at 300km, Which it was and then at 5000km. How often do you guys do your diff fluid? Is this preventative maintenance or just a waste of time and oil?