Manual transmissions vary the speed and torque applied to the rear wheels by allow the driver to select different gears. Orderly progression through the gears permits optimum power delivery, and smooth transmission operation is essential to timely gear selection. Proper lubricant qualities and levels are fundamental to transmission function and longevity.
A manual transmission that is noisy in neutral may need fluid or an input shaft bearing. The bearing is designed to be in contact with lubricant, so low fluid levels will result in the same sounds a defective input shaft bearing makes at idle. Loud operation in all forward gears may also be due to insufficient lubricant levels or quality. Top off or replace insufficient lubricants to quiet transmission functions.
Low fluid level or poor oil condition can cause harsh engagement of forward gears or prevent gear selection at idle. Insufficient oil can also cause the transmission to "hang up" in a particular gear and impede any shift changes attempted. Temporary relief from this situation can sometimes be had by shutting off the engine and allowing the transmission to cool. Metal parts expand and bind when overheated and lack of proper lubrication creates excess heat.
Lubricants that exceed viscosity weights specified by the manufacturer will result in sluggish gear selector action and slow component function. Overfilling a transmission can silence defective components temporarily, but the excess oil will eventually escape. Overages will be expelled through a vent in the transmission case, but seal failure may also occur. Transmissions that appear to be leaking may be exhibiting symptoms of an overfill. Replace affected seals to ensure integrity.
Seals ruined by overfills can allow oil to contact the clutch disc and ruin the clutch components. Sophisticated internal parts can become fused by heat and friction resulting from insufficient lubrication. Manual transmissions execute complex actions at high speeds; particular oils are specified at certain levels by manufacturers to accommodate the components that provide ease of operation and reliable function. Consult the vehicle owner's manual for specifications that apply to your make and model.