Which Gear Oil for my Manual Transmission

GEAR AND SYNCHRONIZER WEAR PROTECTION Most manufacturers of manual transmissions and transaxles recommend an 80W or 90W GL-4 lubricant. GL-5 gears oils which are required in hypoid differentials are not used in most synchromesh transmissions. The chemicals used to provide the extreme pressure protection can be corrosive to synchronizers, which are commonly made of brass or bronze. Typically, the use of a GL-5 lubricant in a synchromesh transmission will shorten the synchronizer life by one half. The extreme pressure requirements of spur gears and helical gears found in transmissions are not nearly as great as found in rear-wheel drive differentials. A GL-4 lubricant provides adequate protection for most manual transmissions, unless a unique design consideration requires the extra protection of a GL-5. The reason that many manufacturers have made recommendations of motor oils or ATFs is that petroleum 80W gear oils frequently do not shift well at low temperatures. Motor oils and ATFs are much more fluid at lower temperatures and they are not corrosive toward synchros, but they provide very poor gear protection. These lubricants provide almost no extreme-pressure protection. In addition, petroleum multigrade motor oils and ATFs have very poor shear stability. The shearing action by a manual transmission on thickeners is much worse than in an engine or automatic transmission. Within 5,000 miles the thickeners can be rendered ineffective and the transmission will be operating on a much reduced level of protection. In hot weather these transmissions will whine and rattle because of poor vibration dampening and metal contact. This is why we recommend and use Red Line MTL and MT-90 to provide the excellent gear protection of a GL-4 gear oil in a synthetic lubricant which spans hot and cold temperatures and will not shear or oxidize with use.