Here's an example of how I approach things like differential fluids.
The Ford specification is WSL-M2C192-A and GL-5, but this doesn't tell you much other than it is rated for clutch-type positraction units. The Ford recommended service interval according to Ford publications is 100,000 miles on the diff fluid. If the system is sealed and no water or debris enters the differential, then this is probably OK.
However, the basic specs for differential lubricant doesn't even scratch the surface of what is required in a heavy duty differential. When I originally built the differential in my Mustang, I used Royal Purple 75w-140 simply because I had it on hand. Here's a shot of the Ford Racing 4.30:1 Gears at installation:
Royal Purple ONLY meets API GL-5 or GL-4!!!!
At 8,000 miles, I changed the lubricant to Red Line, again because I had it on hand. Redline is a little better, meeting API GL-5, GL-6, MT-1, MIL-L-2105E and Chrysler spec MS-9763.
However, the best quality lubricants meet and/or exceed far more stringent qualifications. There are only FOUR compaies producing a hypoid/amboid axle lubricant that exceeds ALL of the following specifications:
- API GL-5, MT-1 Limited Slip, Extreme Pressure Hypoid Gears
- MIL PRF-2105E, MIL-PRF-2105D, Mack GO-G, GO-H, GO-J, GO-J Plus
- Mack GO-J PLUS
- SAE J2360
- Eaton Roadranger E500
- Scania STO 1:0
- ZF TE-ML 07A/08
- Dana SHAES 429 Rev. A, DANA SHAES 256 Rev. C
- Meritor O-76N
- MB-Approval 235.8
- International TMS-6816
- ZF TE-ML 05B/16F/21B
- ZF TE-ML 12B
- Voith Turbo 132.00374401 / 132.00374402
- U.S. Steel Specification 224
- Ford WSL-M2C192-A (75W140)
- Meritor 0-76M (75W140)
The one I chose was Ultra1Plus 75w-140 Synthetic made in Houston Texas. This lubricant cost $40.00 for (3) litres ($13.30 per litre) and exceeds all the specifications listed above.
Amsoil cost $22.00/litre, RedLine is $17.95/quart and Royal Purple is $23.99/quart and yet they do not surpass these specification.
Meritor 0-76M alone requires a lubricant that can go a whopping 300,000 miles between changes.
These high quality lubricants are essential to long life of gear driven units....
My runner-up was
TRIAX Synergy Gear MAX 75W-140 GL-5, PAO Synthetic Long-Drain Axle,