2000 323ci trasmission oil

[Mechina]

I have a 2000 323ci with a 5-speed manual transmission, which is making this horrible sound when I change gears so I want to change the transmission oil. I drained it out today and it was only about 750 ml. I called the some local dealer and they said they had oil for it but 2 l. will cost about $80. I was wondering if you guys know where I can buy that oil for less and also if you can recommend some oil addons that will improve shifting and something to care take of that noise. Thanks.

[AlpineSixAndSeven]

Some people have used Royal Purple Syncro Mesh with success Or another Company called Redline

[Mechina]

I changed the transmission (5-speed manual) oil using Royal Purple 75W90 and it took about 1.5 liters. When I start up the car and let go of the clutch it starts to make a light buzzing sound. When I'm driving the car on first, second, third, and fourth gear when I press down the gas it starts to make a loud buzzing sound but it doesn't do it on fifth. When I am not on pressing down the gas it doesn't make that does. I'm thinking the problem is the transmission so do you guys know any fluids I can put in to get rid of that sound? Or could it be something different?