foamy diff oil

[05mxdiesel]

So I finally got to changing my diff oil today at 15,800 and was so suprised to the conidtion of the oil in the rearend. After an hour drive home and the rearend was still warm I tore into it to find frothy and foamy oil. I was just wondering if any of you noticed this as well. The front was a lot cleaner though and I replaced with amsoil severe gear 75w/90. Rear took 3. 5 and front took 2. 5 using the 1/4 inch rule. I hope the amsoil doesn't foam like the stock oil.