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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.
Using a thicker (higher viscosity) PETROLEUM oil (than the stock 80, 90, or 80W90) ending in 120 or 140 or 145, will take the transmission operation out of the design operating area, as far as gears & parts speeding up & slowing down during shifting due to oil friction, etc, thus gear changing up & down can be different, and may give problems.
I recommend that you NOT use any additive if your transmission is filled with synthetic oil.
One such additive is made by Dow Corning and sold, $$$$, by bearing sales companies.
I think they are now good enough for me to recommend them in general of dino oils.
In some instances seals & the synthetic oils may not be perfectly compatible.