What Exactly is Hypoid Gear Oil and Do I Need to Use It in an 8.8 Diff (’92 Ranger)?

[Captain Ledd]

Oh yeah, it's perfectly fine.

These types of gears haven't changed much since the 1930's, I mean things have gotten a little tighter and more consistent/better manufacturing processes, metallurgy, etc, but it's still just a spiral bevel gear that's offset from centerline. It's not a racing setup that needs to handle some extreme of stress, it doesn't sound like you're doing extreme amounts of towing, it probably has some miles on it. What I'm getting at is that while it's cool to have the really nice stuff, most things just don't need it. The more important part is that it's kept full and relatively clean and it'll have a plenty long service life. In most cases it'll long outlast the vehicle.

These axles, the 7.5" and the 8.8" are pretty stout axles all in all. Only the spider gears in the 7.5" popping under higher power is the only real main outright weak spot. And even then, there was a member a while back that built a NICE 9" unit for his desert truck, but decided to weld up the spider gears 7.5" just to see how long it would last. After something like 2 years, he ended up just swapping out the axle to put the 9" in. Never ended up breaking it.

 

[rangerenthiusiast]

Oh yeah, it's perfectly fine.

These types of gears haven't changed much since the 1930's, I mean things have gotten a little tighter and more consistent/better manufacturing processes, metallurgy, etc, but it's still just a spiral bevel gear that's offset from centerline. It's not a racing setup that needs to handle some extreme of stress, it doesn't sound like you're doing extreme amounts of towing, it probably has some miles on it. What I'm getting at is that while it's cool to have the really nice stuff, most things just don't need it. The more important part is that it's kept full and relatively clean and it'll have a plenty long service life. In most cases it'll long outlast the vehicle.

These axles, the 7.5" and the 8.8" are pretty stout axles all in all. Only the spider gears in the 7.5" popping under higher power is the only real main outright weak spot. And even then, there was a member a while back that built a NICE 9" unit for his desert truck, but decided to weld up the spider gears 7.5" just to see how long it would last. After something like 2 years, he ended up just swapping out the axle to put the 9" in. Never ended up breaking it.

Excellent. I really appreciate you taking the time to give me so much good info, Captain Ledd!

 

[don4331]

Quick science lesson:

Sulfur in diesel, greatly improves it lubricating properties*. However, when sulfur is subject to high temp/pressure of combustion, it combines with oxygen to create sulfur dioxide (SO2). When sulfur dioxide meets up with water, you get, SO2 + H2O = H2SO3, sulfuric acid aka acid rain. Acid Rain being very bad for flora and fauna.

So, EPA required sulfur qty to be greatly reduced, more refining = higher cost diesel.** But we really shouldn't be burning the oil in the differential, so little chance of creating sulfuric acid, so EPA still allows sulfur to be added to gear oil to improve its lubricating properties. Just don't burn your used gear oil, take it to recycler.

Zinc in oil helps with ensuring lubrication during the immediate start up of engine - when there isn't yet oil pressure. But when zinc (in the oil) squeezes past valve guides/rings and travels through exhaust to catalytic converter, it causes the early demise of said part. So, the EPA doesn't dislike zinc; it's automakers can't pass the long term testing with significant amounts of zinc in oil, so they specify a low zinc oil. Note: The initial oil in engine contains much higher amounts of zinc to allow successful break-in of parts.

In the differential, in most cases, the ring and pinion are already coated in oil, so they don't need the zinc.***

Phosphates (P04) are similar to sulfur, they make over "slipperier", but when subject to high temperature and pressure, they combine with Hydrogen, to create H3P04 aka phosphoric acid. Again, long term exposure of catalytic converter to phosphoric acid degrades its performance. In order to meet emissions tests, manufacturers need the catalytic converter, so low phosphate oils are in order.

Again, in differential, we aren't burning oil, so no issue with higher phosphate levels in gear oil.

Offset spiral bevel, aka hypoid, gears need the additional lubrication of phosphates and sulfur as the teeth are sliding as they are rotating.

Hopefully, Google doesn't contradict me.

*Gasoline doesn't have useful lubricating properties, so sulfur in gasoline serves no purpose. Therefore, gasoline has never contained significant amounts of sulfur.

**Like removing lead from gasoline, removing sulfur from diesel in older engines has very bad effect. At <15 ppm, diesel loses its lubricating properties, it becomes more like gasoline. So, ULSD is allowed 15ppm; well below the LSD value of 500pm.

In Europe, you can get no sulfur diesel - the new electronic fuel injection systems don't need the lubrication. Just don't put it in your old diesel or you will be buying a new injector pump.

***High pinion differentials shouldn't have significant load applied until they have turned a few revs. But most of us don't drag race off driveway, so not an issue.