I took apart my RTR diff and it has that grease set up. I was thinking of putting diff oil in it now. Is diff oil like a liquid? and if so, how will it stay in the plastic casing for the diff? It looks like it would just leak out.

Some diff are sealed with O-Rings. Some are not sealed which means only grease can be used. Yes, diff oils are like cooking oils, they are liquid.

If your diff is not sealed, diff oil will not work because it will leak.

What car do you have?

The oil used for a diff is usually more like honey than cooking oil. It is very thick.

but either way, they will leak if the diff is not sealed.

the diff I took apart is a MGT 3.0. but I was thinking of putting diff oil in a new buggy (Ofna Picco)

I believe the MGT 3.0 has sealed diffs, not too sure about the Ofna Picco