1995 Explorer 4D4WD, conventional rear end, Axle Code 45. I'm preparing to
change the differential lube at 100,000 miles.

I have a Shop Manual which is causing me some confusion. In the servicing
section (00-03-30) it specifies XY-90-QL, Ford Spec ESP-M2C-154-A. I believe
that is a synthetic oil.

In the maintenance section (05-02C-9; 05-02B-43), it specifies XY-80W90-QL,
Ford Spec WSP-M2C-197-A. I think that is petroleum based.

I would prefer to use the same type and have heard stories of rear end
damage from mixing/changing lube types.

Does anyone know if or when Ford went to synthetic gear oil?