It's just the planet gears / CWP were never designed for the abuse they are getting. The only thing you can do is get the best second hand matched CWP pair you can find, get them peened and polished, and then fit on a Quaife / Gripper diff. In my experience (tested on a 2wd ATB with 391bhp to the rear wheels), unless it is a gravel spec rally car, the oil doesn't get hot enough to warrent a cooler.