7/18/2011 7:07 PM
This is a 460w gear oil (same family as 90w, AGMA 7 spec to be exact). The reason for such a high viscosity is due to the style of gearbox, which has a worm gear. The input/through shaft is the worm (similar to a screw) and the pump output drives off of this worm. There is an incredible amount of friction potential with this style of gear box, and I suspect a synthetic is prescribed due to the high temps possible with this style of gearbox.
The only drawback; while the pump side seal is a Viton material, the input (worm) shaft at the front and back are rubber. Some synthetic chemistry is not so rubber friendly, so possibly this has some effect on some of these boxes.
GM trucks have had a similar issue in the past decade and a half or so. Forever, in front and rear diffs, the recommendation was to fill to the bottom of the fill plug. Then all of the sudden, the recommendation was to fill an inch below the plug... why? Because the front and rear axle seals were failing at a high rate, depending on chemistry involved when the oils were changed or topped off. Top offs are especially crucial, because the chemistry must match, or incompatibility issues can arise, which include seal failures.
Not saying this is always the issue, but something to be aware of when spec'ing your replacement oil for this gear.
Greg Dinger
970-903-2296