Difference between 85W90 and 90W gear oil

[Kestas]

Is there really a big difference betewen 85W90 hypoid gear oil and straight 90W hypoid gear oil? I mean do these five points really make a big difference in gear oil performance? Why bother with additives for such a small heat thickening effect? It seems as funny to me as would 25W30 motor oil.

 

[MolaKule]

It depends. Some of it is specmanship and some of it is real.

If you have a full synthetic 90 weigh gear oil it will protect and wrok from 70 to 95 weight.

If it is a dino oil, the VII's are needed to make it swing from low to high viscosity, depending on the viscosity spread needed for gearbox.

Most 70W90's should fill the bill for this viscosity.

 

[par]

My diff call for 80w-90 or straight 90. I tried 75w-90 synthetic and really didn't like the low 'hum' that resulted. I went back to 80w-90 synthtic blend and it just seems quieter to me. The truck only has 20k miles so I doubt if it's wear.

 

[widman]

SAE 90 is a very open spec. A big range. SAE 85W sets stricter limits on it's thick side.