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I used to own a 1968 Volvo 144. I changed the differential oil in it to 75W90 synthetic. I don't know what I drained out, but it was definitely thicker than the 75W90 I put in.
Then the car developed a loud gear rattle at part throttle at 65-70 MPH, my normal long trip speed at the time. I believe that that rear end needed a slightly thicker oil. Any mileage improvement was very minimal, well under one MPG.
Hypoid gears need a minimum oil viscosity to prevent metal to metal contact.
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