I think it is worth it and the one thing that I have seen in action is that it retains viscosity better than other oils. The color test does not say much and I would think that short of contamination, it would have easily made 100K.
I towed heavy in my 2010 F-150 FX4 with a 9.75" rear axle and after 80K miles the Amsoil had retained 94% of it viscosity versus Motorcraft retaining 89% of viscosity at 56K miles. The Amsoil had a TAN reading of 4.3 versus a virgin TAN of 3.1 so still very healthy with many miles of life left (I traded it at 160K miles for the 2015 PSD I had).
While I am somewhat skeptical of Amsoil engine oil and transmission fluids, I can say that I believe their gear oil is arguably one of the best on the market. It is also not overpriced like other boutique oils and the extra cost is offset given the typical life of it.
As an aside, when was the oil first changed? I assume the Amsoil was the second change?