Rating Newest Oldest. Best Answer: A manual Transmission will use Gear oil or transmission fluid. and an automatic transmission will use Automatic transmission fluid. Gear oil is a very thick type of oil (engine oil is 30 weight and gear oil is 80 weight) that is thrown around inside the transmission when the gears are turning. Automatic transmission fluid is a Hydraulic fluid that is designed to be pumped and it operates several clutches band brakes inside the transmission.