One word of caution regarding redline and synthetic oils in
general.
In a spider transmission you want to use MT-90. This is the
same oil as the 75w90 but without the friction modifiers
needed for a limited slip diff. These friction modifiers
make the oil more 'slippery' so to speak. This makes the
oil too slick and as a result does not allow the
synchronizers to work properly.
I've used MTL and MT-90 in alfa transmissions (and many
others) with very good results. However the MTL seems a
little too light for an Alfa so the MT-90 is a better
choice.
If the synchros are in good shape to begin with there
should be no problems. If they are on the fringe it might
help. If they are gone, most likely nothing short of an
overhaul will help a tired gearbox.
No problem with the 75w90 in the diff if you want to use it
there.
HTH,
Jeff
Thanks!