I also have NO objection to Castrol 80W90 GL5 oil in -synthetic; NOR do I have any objection to any of Spectro's GL5 rated gear oils, in appropriate SAE viscosities.
Any oil will thin & thicken with temperature changes on 'its own chart curve'.
If you change from a dino to a synthetic, & get some weeping or leaks, change to another type of synthetic, or go back to a dino.
With the change back to dino oil, the seal(s) are then likely to reshape with some miles & time and stop weeping.
I do not recommend ANY additives except in very special circumstances.