LittleStevie wrote:Unless you're a gear designer with a PhD in Mechanical Engineering, stick with what the Nissan engineers spec out in the manual.
Problem with that is when the Nissan engineer spec'ed 75W-90 gear oil for the rear diff in 2004 or earlier he was using the old SAE designation which had a cSt@100°C viscosity range from 13.5 to 24 with most 90W gear oils being made at that time at the midpoint of about 18 or higher cst's, old designation shown in the following table;
Since Jan 2005 SAE came out with new J306 gear oil designations, as in the following table;
http://www.finalube.com/announcements/2 ... y_2006.pdf
...which downgrades the 75W-90 oil to a range of 13.5 to 18 cSt@100°C and since about 2006 most 90W modern gear oils being made in the midpoint of about 15 cSt's. There aren't many gear oils being built yet in the 75W-110 range but these might help with high wear metals showing up if one was doing in use gear oil analysis.
Here's an interesting white paper from Amsoil showing comparision of some 75W-90's and how various stand up in testing;
http://www.amsoil.com/products/gearlube ... Paper.aspx
Interestingly the new Amsoil 75W-110 has a cSt@100°C of about 20 which means it falls well within the old range designation of a 75W-90.
http://www.amsoil.com/storefront/svt.aspx
Somewhere I've seen some comparision charts of cSt@100°C related to oil film thickness which would give some further wear information.
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