gearbox and diff
 

i'm going to change the gearbox and diff oil im my 04 sti tommorow
is castrol syntrax 75w90 a good oil for box and diff?
the box looks easy job drain and refill with 4.1l
the diff looks easy to only problem i have is how do you know its full? and how much does it take? also is oil same grade as box?
thanks

for the rear diff i squirt it in until it comes out the top plug hole. I thought that bottom hole is drain, top is fill.