Overfilling a differential?

I was skimming through kyosho's instruction manual for the mp777 sp2 and it stated "fill up to 80%". It diagram does not show the last gear in there, so maybe they just don't want you wasting fluid?
Essentially, I am not used to fluid filled diffs (my other RC is a t4), so I wanted to make sure if I rebuilt the diff properly.... Is it suposed to be full? or not? or does it not matter?

Thanks

[Arct1k]

Does really answer it but worth a read

http://www.rc411.com/pages/howto.php?howto=24

[BrianG]

The last gear does take up some room, so 80% sounds about right. I usually fill my diffs until the fluid just covers the spider gear shafts. I also spin the gear a little to remove any air pockets, which would make the fluid level fall a little.

Many diffs have a set screw at one end (for the gear pin) - maybe you could use that as a fill point? Wouldn't have to worry about the 80% thing that way...

What I learned was the more diff fluid you put in, the tighter the diff feels, I always topped mine off 99% full

[pipeous]

you need to leave expansion room for heat. 80% allows the dif fluid to expand. if you fill them full then when it expands, it pushes out past the seals. like above, just fill to the cross pins and you'll be fine. if you need it tighter, put thicker oil in