I take it your car is a 5-speed. I suspect changing the transaxle gear oil will not solve your problem.
What is your present mileage? Has the "gear oil" been replaced before?
If its a manual, use only GL4 75W90 or 80W90. Do not use GL5 or a GL4/GL5 blend.
I had mine replaced at a Mr. Lube about 6 months ago for less than the cost of an oil change. I would just as soon them do it and dispose of the old gear oil for the cost involved. They will also monitor gear oil level periodically for you for nothing, which has benefited me as I have a left tranny seal leak.
You should buy a Haynes manual.