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Old March 30th, 2006, 19:25   #103
Join Date: May 2001
Location: San Antonio, TX

You've got it, Rob -- synchromesh isn't 75W-90 - it's an application specific gear lube that replaces 5W-30 engine oil in certain GM and Chrysler transmissions.

75W vs 75 gear lubes - apples and oranges:
There were questions about 75W and 75W-90 earlier and it seemed that things got off-course a bit. There are a couple of different types of viscosity measurement. Viscosity is generic 'thickness' of the fluid in a specific condition. The SAE 90 (the -90 side of 75W-90) is kinematic viscosity at 100C. One way of measuring this is to put oil in a calibrated cup with a hole in the bottom - you time how long it takes the fluid to drain and look up the viscosity on a chart. Other viscosity measurements are made with the fluid in motion - like HTHS (yes Rob - I agree it's a viscosity measurement ) where the 'apparent' viscosity is measured as a fluid lubricates a bearing at a certain speed and load and temperature. This gives a different value than the kinematic value, even at the same temperature, because parts of the oil - like the viscosity improver additives - 'straighten' or 'align' when being squeezed thru narrow passages and the oil temporarily 'thins'. The 'cold' viscosity - the 75W part - is an 'apparent' viscosity measurement at some cold temperature. In engine oil, the point is to examine the oil's resistance to cold cranking.

The 'oil in motion' equipment is calibrated with straight-weight reference fluids - they know that a straight 10 fluid causes x amount of torqe loss at a specific condition, and 20 fluid causes more. The oil under test is run, the torque loss is measured, and the test oil is labeled according to how well it behaves compared to the straight weight reference fluids. It's possible for the same fluid to have different ratings as well -- AMSOIL's marine gear lube is a 75W/80W-90 -- it behaves as both a 75W and an 80W in the individual tests.

The point of this is that the 75W and the 90 are different measurements at different temperatures using different measuring equipment and can't be directly compared.

AndyH is offline   Reply With Quote
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