Every time you go around a corner the sure grip clutches are wearing and leaving debris in the gear oil. For cone sure grips that debris is metal. I use a suction gun to suck the gear oil out of my 8 3/4 cone sure grip and I can see the metallic tint in the removed gear oil. So changing gear oil is preventative maintenance.
I get that, but it all settles right there in the bottom. It's easy to get to. If you are seeing metal in the gear oil, there's a problem. The clutches are lined with fiber material. That's the wear surface, not the metal.