Truck Forums - Changing front and rear differential oils.

1999 GMC Sierra 1500 4wd 5.3L.

The Haynes manual call for synthetic in the rear diff, the tag says 15-40mm (0.6 in to 1.6 in) below the hole for synthetic, how do you tell when it is at that level?
https://cimg7.ibsrv.net/gimg/www.tru...5adc731bde.jpg

Is this also true for the front diff when using synthetic oil it it? < Haynes manual says bottom of the fill hole, but I’m assuming this is for regular oil since the manual does not call for synthetic in the front.

Also in the manual there are no capacities listed for front and rear diff, only the transfer case.