See below for a portion of one of the best articles that I've run across on GL4 lubes and Castrol Syntorq. I use Syntorq in the NV4500 in my Dodge truck. As noted it appears that with lower torque / usage your needs will be less demanding and you can get by with a wider range of products with suitable drain intervals.
http://forums.corvetteforum.com/showthread.php?t=995957
Is this risk really worth the reward? Brown continues; “Since applications calling for a GL-4 oil don't need the EP performance of a GL-5/MT-1 oil, the very real possibility for yellow-metal corrosion is not worth the risk. We suggest that customers follow the OEM lubricant recommendations for their equipment. If an OEM recommends a GL-4 lubricant, we would not recommend using a GL-5/MT-1 lubricant for the application.”
In spite of NVG’s lack of cooperation, we do know Syntorq LT is a very good gear lube, compliments of Castrol USA. Lubricants are often designed to provide a viscosity that is low enough for good flow characteristics in cold weather and high enough to provide adequate film thickness and lubricity in hot, high-severity service. When this hot and cold performance is required, a small response to changes in temperature is desired. The oil industry expresses this response as the V.I. (Viscosity Index). From the Data Sheets of Syntorq we find that it has a relatively high V.I. of 166. A high V.I. also indicates it is a better quality base stock to begin with. Lubes that have a high V.I. also have a lower sulfur content. Sulfur is also part of the corrosive equation that we need to avoid. Multigrade oils were first additized with polymers to increase the V.I. of an oil. Additives do not last the lifetime of the base oil, so the benefit of having a high V.I. without the additives are obvious. I also found out Syntorq is uniquely polymer free.
Specification SAE J306 was revised in June 1998 and became mandatory in January 2000. This is the first time that an SAE gear oil specification has had a shear stability requirement. It must meet the 100degC kinematic viscosity stay-in-grade requirement after a 20-hour KRL Shear Stability Test. It is quite possible many gear oils may fail to satisfy the new shear requirements due to a variety of reasons; for example, being formulated too close to the low end of the 100degC viscosity limit, or using a pour point depressant or viscosity modifier with insufficient shear stability. You guessed it, Castrol claims Syntorq meets SAE J306 too! And before 1998, Castrol used the Kurt Orbahn Shear Stability test on Syntorq to qualify its performance.
Trust me when I say my entire goal was to research a viable alternative to the OEM recommended lube. I had trouble finding any gear lubes even in the same viscosity grade. I only found one lube that even comes close in rheological terms and it wasn’t a GL-5 but a synthetic GL-4 specifically made for synchronized transmissions. Remember that the NV4500 is splash lubricated. There is no oil pump or filter in this unit, so the viscosity of the lube is extremely important and not all 75w90’s are created equal in that respect.
As far as base stocks, EP additives, buffer compounds and who uses what; knowledge is pure profit in the science of the lubrication industry and no one is sharing. There might be an equivalent gear lube at a more reasonable cost, but without a degree in petroleum engineering and the resources to test it, I won’t find it before the new GL-4 “upgrade” category comes out.
Syntorq is specifically tailored for the NV4500, no 75w90 GL-5/MT-1 I looked at even approaches it in rheological terms. It meets the new SAE J306 shear stability test. It has a higher V.I. than any of the GL-5/MT-1 alternatives I looked at and is polymer free. The NV4500 doesn’t need the extra protection of a GL-5, so why take the risk? It’s my opinion that Syntorq is the “Holy Grail” of gear lubes for our beloved under engineered NV4500. Unfortunately, Syntorq LT also comes with the “Holy Grail’s” price too! Well, what should you do? Use a less expensive GL-5/MT-1 lube (change it frequently to mitigate the risk), use the expensive Syntorq and be sure, or wait for the new GL-4 “upgrade” category to see what life brings us? Me? My only temptation would be to try a qualified synthetic GL-4 specifically made for synchronized transmissions, but I’m more inclined to just bite the bullet, pay for the Syntorq and not worry about a thing…
Your best dealer choice for Syntorq is General Motors. Ask any GM dealer for P/N 12346190. It will be substantially less than any Dodge dealer will. You can also obtain a wholesale price for Syntorq plus shipping charges from Standard Transmission of Texas.
