What's the best synthetic fluid for the rear differential?

I just did the front and rear diffs. I used Royal Purple 75w90. To be honest it probably didn't need it. The fluids that I drained out we're pretty clean and could of lasted until 100k. But I just did it for a piece of mind. Noticed no difference but knowing it has fresh fluid makes me feel good.

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SCY7H3 said:
How did the magnetic bolt look? What color was the oil?

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The magnetic bolt in the rear had quite a bit of gunk build-up on it. The front wasn't so bad. The gear oil came out pretty clean. Looked just like fresh gear oil. I rubbed a little between my fingers to see if it was still thick and lubricated well and it felt pretty fresh.

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Hrm, are you the first owner of the vehicle and is this the original oil? Normally at. This point the oil should look grayish yellow

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Hey guys I got quoted from a company and they were going to replace the differential fluids with redline 75w140. Should I be concerned? Should I ask them to use 75W90?
 
Someone else might want to jump in on answering this but I'd probably stick to the OEM viscosity. Not sure if you need the higher one especially in synthetic and how thicker oil will react with the diff and plates inside once it warms up.

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SCY7H3 said:
Hrm, are you the first owner of the vehicle and is this the original oil? Normally at. This point the oil should look grayish yellow

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I am the second owner. The previous owner leased it until 20k miles. I highly doubt the diffs were changed.
The gear oil wasn't perfectly clear but looked pretty good for having 60k miles on it. I wish I had snapped a picture while it was draining out.

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I think the clutch material is magnetic so pretty much all that debris sticks to the magnet. So that's why the oil doesn't get contaminated. The thing though is the color won't change much in a diff as it would with something that burns the oil like an engine. The oil would break down in certain ways but the color wouldn't shift much.

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SCY7H3 said:
Someone else might want to jump in on answering this but I'd probably stick to the OEM viscosity. Not sure if you need the higher one especially in synthetic and how thicker oil will react with the diff and plates inside once it warms up.

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For the record the OEM viscosity is actually 75W140. That is what the OEM fluid in the car is actually rated at and that is what the OEM fluid that Nissan sells is:

http://www.thenismoshop.com/Factory-Nissan-Fluids_c_995.html
 
That viscosity is for the first gen? Second Gen viscosity is 80w90 non synthetic or 75w90 synthetic. Fxlr8 posted this for the second gen: ///5890

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SCY7H3 said:
That viscosity is for the first gen? Second Gen viscosity is 80w90 non synthetic or 75w90 synthetic. Fxlr8 posted this for the second gen: ///5890

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Interesting, I guess the Nismo shop doesn't sell 2nd gen fluid. I assumed since there was only one type/viscosity of gear fluid that both generations used the same fluid, but I guess they just don't sell the correct fluid for the 2nd gens.
 
They probably just sell race grade oils

I checked the first gen service manual and they specify API GL-5, SAE 80W90 oil in both the front and rear diffs. The 80W specification is when the oil is cold, the 140 weight specification is when the oil warms up, it thickens to a 140 viscosity. Plus when you're racing, you'd need that as the 80W90 oil would break down faster and wear the internals.

For the second gen, the 75W90 Synthetic spec is recommended when putting on a tow package(hitch). That's because there would be more wear on the rear diff when towing and instead of increasing the viscosity to 140, they require the use of a synthetic as that lowers the friction substantially lowering operating temps.
 
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