4 speed oil change

wanted to change out my oil i had put in 80 90 valvoline limited slip some time ago
was wondering what oil is best? i sometimes have some second gear grindings when flooring it should i go thicker? maybe it would help? i live in Ca. any advice is welcome the a833 has some miles on it that why i wanted to see how thick i can go.

another thing adding oil to my trans from underneath would i use a syphon pump?