Back Hoe 416E

Hie... I have a CAT back hoe. When ever I do an oil analysis of the rear and front differential system, the viscosity is very low compared to the oil I use. I have changed the oil at stipulated intervals. I even sample the oil at quarterly and half interval but the viscosity I obtain is very low for 85W140 grade oil


What maybe the cause of this sharp drop in the viscosity as soon as I change the oil?