Transmission Oil to Use

Picking the right transmission oil can be tricky. The problem is most oil sold in the store are now are high performance GL-5 oil. Many manual transmission cars require GL-4 oil. With some cars, transmission does not shift right even when the recommended oil is use. To understand the difficulty in selecting transmission oil, is necessary to understand how a synchrnizer work. Synchronizer was invented by Dr. Porsche. It's purpose is to spin the gear up to speed before engaging to prevent gear clash. At the heart of the synchronize are pointed rams that has to push past each other before gears can engage. Because the rams are at an angle, how hard it is to push out of the way depends on friction between the rams. If oil used is very slippery, rams can be pushed out of the way easily, and result in gear clash. If oil is too high in friction, then is hard to shift into gear, and people call that "notchy" shifting. The shifter seems to hang up before going into gear. With the perfect angle and just the right fluid with the right friction, shifter will go from one gear to another like hot knife through butter. Therefore, transmission fluid has to match up with the design of the transmission for perfect performance. Even if fluid is perfectly matched, cold start in cold weather will mess up all the parameters again. Some people use synthetic for cold weather, or use a thinner fluid like the GM Synchromesh talked about further down.

Second thing to consider is GL-5 coats the gear with anti-wear additive. Many synchronizers are made from brass. The anti-wear additive adhears too well to the brass, and tears it up resulting in wear. GL-4 is typically the one to use to protect the synchrnoizer. GL-4 fluid also has more friction for the rams to work right. GL-5 is too slippery, and results in clash.

How Synchronizer Work:

Oil to Use:

Summary:

-If manual calls for GL-4 fluid, absolutely use GL-4 fluid. DO NOT use GL-5 fluid. It can damage the transmission.
-If manual calls for GL-4/GL-5 fluid. DO NOT use GL-5 or EVEN GL-4/GL-5 fluid. Only use GL-4 fluid, or there can be transmission damage. Don't be fooled, even GL-4/GL-5 sold at typical auto parts store is not good for manual transmission. Transmission will likely clash during shifting.
-If manul calls for GL-5 fluid. DO use GL-5 fluid.

Do NOT use these oil on non New Venture transmission except to fix shifting issues:

Use these GM fluid only if there are shifting issue, or you want the max shifting performance at the risk of using incorrect fluid.
GM has the following two types of transmission fluid below. One has friction modifier, and the other does not. The viscosity in them are similar to 70W-80, so is lighter than 75W-90. Typically, 75W-90 (14.5 cSt) is recommended for FWD transaxel that have spider gears in the differential. 70W80 fluid are thinner 10.5 cSt lubricants, and are for RWD transmissions.

GM 12345349 The new number is: 88900333 'Synchromesh Transmission Fluid' ~$9/qt - This one is without friction modifier. This one has been recommnded as a cold weather replacement for 75W-90 GL-4 transmission fluid where the thicker 75W-90 fluid will have rough shifting. The MSDS sheet list the fluid as Shell MTX Fluid. This fluid is used even by some Honda and Mazda dealer.

GM 12377916 The new number is: 10953509 or ACDelco 10-4014 'Synchromesh Transmission Fluid Friction Modified' ~$12/qt This one has friction modifier for certain transmission that has friction lined synchronizer rings. Originally made for NV1500 and NV3550, but highly recommended by many in the forum for othere non-GM manual transmission cars. Equivalent to 5w-30 engine oil. Note that GM fluid is twice as expensive as the Pennzoil Synchromesh Fluid, but they are the same thing. GM fluid is outsourced to Pennzoil anyway. In most auto parts store, you will find the Pennzoil, but not the GM part number.

Use these GL-4 Transmission Fluid Part Number if you Can't Find GL-4 Fluid

GM 12346190 ACDelco 10-4059 or Mopar 4637579 'Synthetic Manual Transmission Fluid' 75W-90 GL-4 ~$20/qt Also called "Syntorque" Also sold as Castrol Syntorque. Made for the NV4500 manual transmission on full size truck. GL-4 rating is for "yellow metal protection". This is the standard GL-4 transmission oil. Is not as good as the Synchromesh above in terms of shifting performance. However, it does have the correct 75W-90 GL-4 rating. Use this if factory recommned a 75W-90 oil, and you are not comfortable with going outside of factory recommendation. I would use this one unless you run into shifting issue, then use the GM synchromesh. Only issues is this one is hard to find, and is a dealer part. See aftermarket fluid below for similar products.

Note:
Viscosity difference between GM Synchromesh and 75W-90 fluid:
75W-90 ---- 40C, cSt=87.6 100C, cSt=13.9
Synchromesh --- 40C, cSt = 41.6, 100C, cSt=9.08
Therefore, GM Synchromesh is much thinner fluid than the 75W-90 fluid. Unless there are issues with shifting, stay with the 75W-90 fluid for Toyota and Mazda and 10W30 for Honda and Accura.

Aftermarket Performance Transmission Fluid

Red Line - They sell two types of manual transmission oil. MT-90 75W90 GL-4 Gear Oil and 75W90 NS GL-5 Gear Oil. Get the MT-90. It has the proper rating of GL-4. DO NOT get the MT-90 if manual calls for GL-4 or GL-4/GL-5. Higher GL number is NOT better.

MOTUL GEAR 300 75W90 - This one is GL-4/GL-5 rating. Would stay away if GL-4 is recommended. Would be suspicious even if GL-4/GL-5 is recommended by the manual. However, there are positive review for Mitsubishi vehicles using this. Just beware.

AMSOIL Manual Transmission & Transaxle Gear Lube 75W-90 (MTG) GL-4 - Has the proper grade, and is high quality oil . Generally recommended.

Royal Purple MaxGear 75W-90 GL-5: This one is a GL-5, but is suppose to not have the harmful additive for the synchros. Generally recommended.

Conclusion

If factory recommend GL-4/GL-5, be sure to ONLY use GL-4 only (hard to find). DO NOT use fluid labled GL-4/GL-5. If factory recommend GL-5, then use GL-5 only. Unless specified, do not use GM Synchromesh without friction modifier or Pennzoil Synchromesh. GM Synchromesh does give better shifting performance, but is too thin if 75W90 is called for. Use GM Synchromesh only as a last ditch method to improve shifting issue, but the thin oil will cause bearing noise in the long run unless the transmission is made for Synchromesh fluid.