Inside the diffs have a oil that comes in a numbered weight. The gears on the outside off the diff usually require black grease for proper operation. I use red and tacky for all my gears and what ever weight oil for the inside of the diffs.
Lower the number, thinner and transfer power across the axle really easy.
Higher number, thicker and sends power more evenly across the axle. Like limited slip or posi.
Any metal to metal gears should have some type of grease on them. Portals included. That’s what the little tube of black grease is for.