GL-4 and GL-5 are the API specifications for gear oil:
- API GL-1, GL-2, and GL-6 are inactive.
- API Category GL-4 designates the type of service characteristic of spiral-bevel and hypoid gears in automotive axles operated under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications.
- API Category GL-5 designates the type of service characteristic of gears, particularly hypoids in automotive axles under high-speed and/or low-speed, high-torque conditions. Lubricants qualified under U.S. Military specification MIL-L-2105D (formerly MIL-L-2015C), MIL-PRF-2105E and SAE J2360 satisfy the requirements of the API GL-5 service designation.
GL-4 and GL-5 products typically use the same extreme pressure additive system, with GL-5 having about twice the concentration of a GL-4. Too much of this additive could cause excessive chemical corrosion of the gear surface. If a GL-5 gear oil is used in a application where GL-4 gear oil only is called for, chemical corrosion of "yellow metal" components may occur, such as bronze synchronizers, brass bushings, etc. This may lead to shifting difficulties or shortened tranny life.
In the R154 and W58 transmissions, either GL-4 or GL-5 gear oil is allowed. In gearboxes where a GL-4 *only* is specified, Wingman is correct...using a GL-5 oil can cause damage to the tranny.
Johnathan - What I'm saying here is using RP Synchromax in a tranny with shifting problems *may* help...
I'm not recommending it for use in a tranny that is functioning normally. Synchromax has a viscosity of 35.3 cst at 40 deg C and 7.7 cst at 100 deg C...Max-Gear 75W-90 has a viscosity of 144.5 cst at 40 deg C and 21.0 cst at 100 deg C...Max-Gear 80W-90 has a viscosity of 150.3 cst at 40 deg C and 16.8 cst at 100 deg C. Both cold/warm viscosities are much higher than Synchromax and illustrate why a GL-4 or GL-5 gear oil is specified for our manual trannys....per the Red Line tech data sheet:
With most lubricated components, the slipperier the better, but this is not so with manual transmissions. The synchronization of shifting gears requires friction to transfer energy from the synchronizer, which is locked to the input shaft, to its mating surface attached to the gear to be locked in as the drive gear. The drive gear is selected by using the shift forks to slide a synchronizer ring, which rotates at the same speed as the input shaft, in contact with the selected drive gear. Once the drive gear is brought to the same speed as the input shaft, the locking ring on the synchro assembly is allowed to slide over and lock into the drive gear. The time this process takes depends on how easily the synchro ring moves and the rate of frictional energy transfer between the two synchronizer surfaces. Higher viscosity lubricants slow the sliding of the synchro ring on the input shaft and require a longer time for the oil to be squeezed out from between the mating synchronizer surfaces. After the lubricant is squeezed out, the coefficient of friction of the lubricant determines the rate of frictional energy transfer between the two surfaces. Slippery lubricants such as hypoid gear oils (like RP Max-Gear) can take too long to synchronize the gears, which promotes synchronizer wear. Red Line MTL and MT-90 has a coefficient of friction which is greater than conventional oils, allowing a quicker transfer of frictional energy.
The cliff notes version is the R154 and W58 trannys NEED a certain amount of friction to work properly. RP Max-Gear is *too* slippery to provide this friction...RP Synchromax is even more so. It is more an ATF type lubricant and thins out significantly at higher temps. IMO, these 2 gear oils are not well suited for our manual trannys...the success many guys have seen using these oils is due to the slippery characteristics of these gear oils "freeing up" (reducing friction) parts in the R154/W58 that are on the way to failure. This is not fixing the problem...that sound from your tranny is the sound of inevitably.
The reason more modern trannys spec a gear oil like Synchromax (or GM's synchromesh) is to improve cold shifts. The thicker oils in a GL-4/5 category do not shift well at lower temps...they need to warm up. The problem is Synchromax does not have the extreme pressure protection, oil film thickness, and shear stability required...this will increase wear on the gears. Synchromax may help quiet down a worn R154/W58 for a while, but it is not really helping the problem...it's masking it.
Just for comparison, Red Line 75W-90NS is a GL-5 and has a viscosity of 95.0 cst at 40 deg C and 15.6 cst at 100 deg C and Red Line MT-90 is a GL-4 75W-90 and has a viscosity of 90.0 cst at 40 deg C and 15.6 cst at 100 deg C. These two gear oils are almost identical in this respect...the difference is the additives...MT-90 is more suited to the R154/W58. The advantage of these are their low temp viscosities...it will help smooth out shifts until the oil gets warmed up.
BTW - there are quite a few excellent GL-4 gear oils out there, all of with will work fine in a R154/W58...gear oils have a much easier time coping with the enviroment they work in vs motor oil. The biggest reason I like the Red line MT-90 for use in a R154/W58 is it's low temp viscosity characteristics. Especially in the R154...it's not the smoothest shifting tranny and will benefit the most from MT-90.
As usual...the devil is in the details