Here is an updated version of the original White Paper:Manual Transmissions and Lubricant effects.
I think manual (or Stickshift or Standard) transmissions are more fun to drive than automatic transmissions. Manual transmissions require more driver interaction than do automatic transmissions. You can’t talk on the cellphone, or eat, or text when you have to shift gears. 
In this paper we examine the internal mechanisms of the manual transmission and the effects of the lubricant’s viscosity and additives. We are discussing light truck and passenger vehicle manual transmissions.
We will not discuss OTR or heavy-duty transmissions which use require a different type of lubricant, such as the MT1 rated lubricant.
A modern gearbox is of the constant mesh type, in which all gears are always in mesh. This constant mesh and the cut of the gears insure a rather quiet transmission. In any one gear, only one of these meshed pairs of gears is locked to the shaft on which it is mounted. The others are being allowed to rotate freely; thus greatly reducing the skill required to shift gears. Most modern cars are fitted with a synchronized gear box, although it is entirely possible to construct a constant mesh gearbox without synchromesh, as found in motorcycles for example.
Some manual transmissions are integrated with differentials to form a “Transaxle.” The differentials here are usually NOT the hypoid types found in larger vehicles, but are of the spider gear configuration. The exception is the Subaru system where the transmission and the differential share a common sump, hence the need for a GL-5 rated fluid.
Going from the top of the transmission case downward, we have the shifter mound which contains the shift lever and linkages. The shifter will have a seal or boot at the top with an additional gasket to keep the lubricant from flowing out when slung by the gearing. Below that are two shafts, one the input shaft and the other being the output shaft. The input shaft is splined to the clutch for power connect or disconnect. The output shaft goes to a universal joint, then to the driveshaft (a hollow “torque” tube), and the driveshaft connects to the differential via another universal joint.
An illustration of a basic manual transmission is found here, so exercise the shifting as we discuss the mechanisms (not a perfect illustration but makes the point):http://auto.howstuffworks.com/transmission4.htm Shifter Assembly:
The gears resting on the top shaft, the input shaft, are locked onto that shaft and rotate at the same rpm as the engine. The bottom output shaft has synchronizers “splined” to this shaft, so they can move around as the gear ratio is changed. The gears on the output (bottom) shaft are allowed to rotate freely on the output shaft or on small roller or “needle” bearings, depending on the horsepower transmitted and the design. The output shaft will rotate at various rpms depending on gear selection. In first gear, for example, you want low output shaft rpm and high torque.
The shifter moves the associated linkage which connects to the shifter forks. The linkages position the shifter forks, and effectively “programs” the shifter forks in order to select the required gear ratio. I.E., for each shift lever position, the shifter forks are moved around to drive the splined synchronizers on the output shaft. The shifter forks have a bore so they can slide on the guide rods. There is a specified clearance between the shifter forks’ bore and the shifter fork guide rods. Lubricant effects:
Too high a viscosity lubricant and the shifting will be hard and sluggish. More force will be required to go from one gear to another. Too thin an oil and the forks will wear, the clearances will increase, and the shifting will become sloppy and uncertain. The correct mix of base oil viscosities is needed here to insure good cold weather and hot weather shifting. Synthetics excel here because of their high viscosity indices.Synchronizer Assembly:
The locking mechanism for any individual gear consists of a collar on the shaft which is able to slide sideways so that teeth or "dogs" on its inner surface bridge two circular rings with teeth on their outer circumference; one attached to the gear, one to the shaft. (One collar typically serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects the other) In our illustration from above, the bottom or output shaft has splines that mate with the synchronizer “collar.” The synchronizer collar moves transversely on the splines, positioned by the shifter fork. When the rings are bridged by the collar, that particular gear is rotationally locked to the shaft and determines the output speed of the transmission by the synchronizer. In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone-shaped brass clutch which is attached to the gear, which brings the speeds to match prior to the collar locking into place. The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings also called blocker rings. Notice, before locking and speed synchronization, a lot of shearing takes place at the interfaces and for the reasons given above. Most synchronizer materials are of brass, but newer synchronizers can be made of strengthened graphite composites. Lubricant effects:
A special Friction Modifier (FM) chemistry is incorporated into the additive chemistry to allow just the right amount of shearing before engagement. I.E., the FM gives rise to a specific dynamic coefficient of friction (COF) to allow engagement without “crunching.” Automatic Transmission Fluids (ATF) DO NOT have these specialized FM’s. Note: the specialized FM used in manual transmissions is NOT the same FM used in Limited Slip Differentials, nor is it the same FM used in Automatic Transmissions. It is important to understand that there are different FM chemistries for different automotive applications!
Bearings: Lubricated bearings are used to reduce friction between rotating parts. The older Munice transmissions, for example, used brass or sintered brass sleeve bearings or bushings. Most modern transmission bearings today, as can be seen by the links given below, are of two main types 1) Roller or needle bearings, and 2) ball bearings. Ball bearings or tapered roller bearings are usually used at the shaft ends to resist radial and transverse loads. Smaller roller or pin bearings are used inside the driven gears that idle on the output shaft. Lubricant effects:
Depending on the horsepower transmitted
and the size of the bearings, viscosities of the lubricants kinematic viscosities range from 6.0 cSt (ATF-range) to 14.5 cSt (equivalent to a light 75W90 gear lube) given at 100C. The anti-wear additives keep wear in check as they rotate in their races. Metal Inhibitors keep any other chemistry from attacking the synchronizers, and anti-rust additives keep any moisture from creating rust on the steel components. For lower horsepower drive trains, the lubricant must be thin enough to penetrate the cages in the pin/roller bearing areas. For higher horsepower drive trains, the lubricant must maintain a thick film in order to protect the bearing surfaces. Of course, the lubricant is also used for cooling. Too thick a lubricant will cause poor cold weather performance and loss of mpg, while too thin a lubricant will cause undue wear. The lubricant also transfers heat from the bearings and gearing to the case.
Gearing: Most gear types in manual transmissions are of the helical type, which because of the cut, reduce noise and vibration. Due to their angular cut, thrust loads are transmitted to the shafts on which they reside. Lubricant effects: Being in constant mesh, they are dipping in the oil bath and slinging the oil up to the shifter assembly. Since they transmit torque, they must have an anti-wear/Extreme Pressure additive in the lubricant in order to reduce wear. The slipping and rolling action of the gear teeth causes localized high pressures and heating. The anti-wear additive forms a protective but complex ferrous film at the contact surface to protect from galling and other wear mechanisms.
Other components such as thrust washers and shims also need anti-wear additives as well. Note: In the past reduced levels of EP additives were part of the MT fluid formulation, but modern formulations use chemistries such as Multi-Function Phosphate esters, ZDDP, metal and rust inhibitors, Viscosity Index Improvers, and synthetic base oils.
Rebuilding manual transmissions usually require only a modest rebuild kit consisting of bearings, synchronizers, and seals unless the transmission has been abused or the wrong lubricant has been used. In that case, gear teeth need to be examined for any chipping, galling, breakage, or other signs of problems.
Here are some individual transmission parts layed out for Jeep transmissions but is typical of others.http://www.4wd.com/Transmission-and-Transfercase/Manual-Transmissions.aspx?t_c=69&t_s=239
Images of Manual Transmissions, both external and internal:http://www.bing.com/images/search?q=manu...ORM=IGRE#x0y810
If you are going to modify or rebuild your Manual Transmission, I highly recommend this book or equivalent:http://www.mre-books.com/transmissions/rebuild_and_modify.html
A variation on the Manual Transmission is the “Automated Manual” using a dual clutch. Some people consider many of the Honda Automatic Transmission simply automated manual’s as well. http://www.allpar.com/corporate/auto-manual-transmission.html
A long winded History and Summary but without the in-depth knowledge of internal
I like this link; it contains online MT manuals for classic Chevy’s:http://chevy.oldcarmanualproject.com/trans/index.htm