sir please change the gear oil right away. fresh oil can do no harm to your transmission. just stick to the grade / viscosity i suggested. fill it up to the mark. btw what car are you maintaining?
incase of any complaint, which i don't foresee, you can always make the oil thicker by adding local available additives. or just simply replace it with multi-grade transmission oil like EP/85w-140.
trust me it's just a myth not to replace the txn oil!