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Recently I changed the oil in my vehicle's rear differential. The manual recommended the use of a "Hypoid" oil, but the guy at the auto parts store suggested a "Limslip" oil instead. I went along with his recommendation since he probably knows more about vehicle maintenance than I do and because the viscosity rating of the "Limslip" oil matched what the manual called for anyways.

Since changing the oil I haven't noticed any issues with my vehicle, and I made sure to take it out for some aggressive driving immediately following the procedure to verify that all was well.

But still I wonder, what is the actual difference between the "Hypoid" oil and the "Limslip" oil? Did I commit some terrible error by using one instead of the other? Is my differential going to explode one day because of it?

Edit: Just for my own future reference; the correct torque for the fill-plug and drain-plug is 30 Newton-meters.

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3 Answers 3

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You should use what your manual specifically says, not the guy at the auto parts store.

Nick C's answer in most situations will probably be correct, that it is just more expensive with more stuff in it that won't apply to your diff and not hurt anything.

However, depending on your specific differential, those additional items could cause problems.

As a point in case, the transmission on my Nissan 300ZX calls for GL-4 gear oil. If you go to an auto parts store and look for GL-4, you most likely won't find any. What you will find is a lot of of GL-5 gear oils that say on the "meets and exceeds GL-4 standards".

This works fine for most cases, but what I found out after running a GL-5 "GL-4 compatible" gear oil for a few months is that the brass components in my transmission will slowly be damaged by GL-5 gear oil as the standard specifies different levels of acidity, mineral compounds, etc and some of those limits will allow for damage to the softer metal components.

When I removed the GL-5 and put in a proper true GL-4 from Redline Oil (they even mention in the product description that this oil is popular on older Nissan, Toyota, etc transmissions) there was a lot of metal sparkles in the old oil. Luckily I pulled it in time to where it seems to have not caused major damage, but had I left it in there for a standard change cycle, it could have killed my synchros.

This is likely not the case on your differential but I'd suggest talking to some enthusiasts of your specific vehicle to find out for sure.

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Thanks for the tip. I checked the datasheet for the oil I purchased, and it says "All Penrite Limslip oils may be be used in conventional hypoid rear axles", so I feel reasonably comfortable that nothing bad is going to happen. The manual calls for a GL-5 oil (well actually it calls for Ford ESW-M2C108A, which appears to be comparable to GL-5), so I'm hoping that the corrosion/pH issue doesn't apply in this case. –  aroth Sep 14 '11 at 13:07
    
Yes, I think you are too, especially because the diff is going to be made out of harder metal than the synchros, but I felt it worth stating as there are instances where using a "newer better product" can cause harm to older systems. This happened with the introduction of ethanol into gasoline also, caused issues with fuel systems that weren't built to handle it. –  ManiacZX Sep 14 '11 at 13:59
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I have used GL-5 in my rear differential for the past ten years and driven over 100,000 miles. I just did a R&R with new GL-5 as recommended by manufacturer. When I examined the old GL-5 , I found no metal flakes in the old as far as I can tell. I recommend following manufacturer recommnedation. No need to try new stuff.

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From what I can tell, Limslip oil is designed for vehicles with limited slip differentials, and thus has extra additives over a normal hypoid gear oil. Hypoid oils have additives to help them cope under extreme pressure, but as limited slip diffs still have hypoid gears in, I would assume limslip oil also has the same additives.

I suspect, therefore, that using limslip in a conventional diff won't hurt it, except that the limslip was probably a fair bit more expensive than hypoid...

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I think you're right, the limslip oil was surprisingly expensive. I wasn't even paying attention to the prices at the time, as my theory was "well the fuel I put in my car costs less than $1.50 a liter, and oil is oil, so this shouldn't be too much more expensive than that". Turns out that theory was very incorrect. –  aroth Sep 14 '11 at 12:08
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