Changing Manual Transmission and Differential Fluids
By Wes Burch
(click photo to enlarge)
Frequency:  Well that depends on how you drive the car.  With "normal" driving, I try to replace manual transmission fluid every 50K miles and rear end fluid every 100K miles.  If you do several track events a year, changing fluids much more often is better.
Tools Required:  In addition to those items mentioned for jacking the car in other areas of Tech Corner you will need:  Something to drain the old fluid into.  In general you will be draining about two quarts (maybe a bit more).  A pump to remove and or replace the fluid can be very helpful (Photo 1).  I got mine at Pep Boys for about $12 and added some 3/8" aluminum tube to the hose ends to help the hose stay in the housing or the new fluid container.
Level of Difficulty:  Easy, other than jacking the car and disposing of the used fluid.

Fluids:  After break-in, I use synthetic fluids.  It is more expensive but synthetics begin breaking down and losing lubrication quality at higher temperatures than non-synthetic fluids.  Also your MPG will be a bit better.
   
C1 to C3 Manual Transmission fluid:  Non-synthetic: Any 70W90 or 80W90 Gear Oil.  Synthetic:  Royal Purple Synchromax.                        
Differential Fluid:  Non-synthetic:  GL-5 70W90 Gear Oil.  Synthetic:  Royal Purple Gear-Max 75W90.  If the car has positraction be sure to add at least 4 oz                         of GM's Limited Slip Axle Lubricant Additive pn 1052358.  EVEN IF YOUR FLUID OF CHOICE SAYS IT ALREADY CONTAINS A POSI LUBRICANT. 		 
C4 (84-88) Manual Transmission fluid:  Non-synthetic: Any 70W90 or 80W90 Gear Oil.  Synthetic:  Royal Purple Synchromax.                        
Differential Fluid:  Non-synthetic:  GL-5 70W90 Gear Oil.  Synthetic:  Royal Purple Gear-Max 75W90.  If the car has positraction be sure to add at least 4 oz                         of GM's Limited Slip Axle Lubricant Additive pn 1052358.  EVEN IF YOUR FLUID OF CHOICE SAYS IT ALREADY CONTAINS A POSI LUBRICANT. 		 
C4 (89-96)  Manual Transmission fluid:  Non-synthetic: 10W30 Engine Oil (GM PN1052931). Synthetic:  Royal Purple XPR 5W30.                        
Differential Fluid:  Non-synthetic:  GL-5 70W90 Gear Oil.  Synthetic:  Royal Purple Gear-Max 75W90.  If the car has positraction be sure to add at least 4 oz                         of GM's Limited Slip Axle Lubricant Additive pn 1052358.  EVEN IF YOUR FLUID OF CHOICE SAYS IT ALREADY CONTAINS A POSI LUBRICANT. 		 
C5 & C6 Manual Transmission Fluid:  You are on your own.  GM says use automatic transmission fluid Dextron III (what I use).  Red Line recommends their                         synthetic Dextron III replacement D4 ATF.  Royal Purple recommends their gear oil Synchromax which is a 75W90 gear oil.                        
Differential Fluid:  Non-synthetic:  GL-5 70W90 Gear Oil.  Synthetic:  Royal Purple Gear-Max 75W90.  If the car has positraction be sure to add at least 4 oz                         of GM's Limited Slip Axle Lubricant Additive pn 1052358.  EVEN IF YOUR FLUID OF CHOICE SAYS IT ALREADY CONTAINS A POSI LUBRICANT. 		 
Until you have done this a few times, think about each step while jacking the car before doing it!  You are playing with a 3,200 lb dead weight.  Even elevated only a few inches it has more than enough stored energy to crush flat both important and unnecessary body parts.
(1)  Get the car up on jack stands as described in other articles of Tech Korner.  It is better to do this when the car is a bit warm because the fluid will drain a bit easier.  But do not install the plugs while the case is warm.  As the case cools and shrinks a bit the plug could end up being too tight.

(2)  Manual Transmissions:  Every manual transmission I've ever encountered has both a drain plug and a fill plug.  (If not, see Differential section).  Remove both plugs and drain the fluid into a suitable container.  Replace the drain plug; I usually add a bit of sealant to the pipe threads.  Use the pump to transfer fresh fluid from the container to the transmission case.  (Photo 2)  Fill until fluid just begins to spill out of the fill hole.  (Photo)  Install the fill plug; again, I usually add a bit of sealant to the pipe threads.  "Brake Clean" spray is a great solvent for cleaning fluid from the side of the transmission case.

(3)  Differential: Differential fluid has a strong and pungent odor.  I suspect that is the case to help non-attentive car owners to wonder "just what is that odor" when their differential is leaking.  When Limited Slip Axle Lubricant Additive is being added, I mix it with the first bottle of gear lubricant.  That makes transfer much easier.            
    C4 and earlier cars do not have a differential drain plug.  I use the siphon pump mentioned above inserted into the fill hole.  To help get fluid from the bottom of the case, I used some 3/8" aluminum tube bent as seen in Photo.  After evacuating as much fluid as possible from the case, reverse the pump and then pump fluid into the case until it just begins to spill out of the fill hole.  Install the fill plug; again, I usually add a bit of sealant to the pipe threads.  "Brake Clean" spray is a great solvent for cleaning fluid from the side of the differential case.           
    The C5 & C6 have both a drain plug and fill plug for the differential.  Remove and replace the fluid as described above under manual transmissions.

(4)  It can be fun to look at the removed fluid.  If you see a lot of shiny particles in the fluid, hold a magnetic above the surface of the fluid.  Observe the shiny stuff begin to move around and make attractive patterns.  Begin saving your money.  

(5)  Lower the car off of the jack stands as described in other articles of Tech Korner.  Dispose of the fluid in accordance with local laws.  This can be tough as many oil recyclers do not want anything except engine oil.