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I am comparing two different transmission fluids that meet the specification for my car.

Two metrics common between them in their specification, are viscosity index and brookfield viscosity @-40C.

The Valvoline Maxlife has: Viscosity Index: 163 Brookfield Viscosity -40C: 9000

Beck Arnley: Viscosity Index: 158 Brookfield Viscosity -40C: 11400 mPa/s

What exactly do these numbers mean? I assume the viscosity @ -40C refers to just that, but does a higher number mean thicker or thinner at -40C?

I have no clue what viscosity index means.

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Brookfield viscosity refers to a global standard method of testing a given fluid for the amount of torque required to turn a gear within said fluid at a certain temperature. The viscosity index refers to the ability of said fluid to flow relative to temperature. With the numbers you provided above the Beck Arney required slightly less torque at -40C than the Valvoline, however the Valvoline had the better flow characteristics. Although it is unlikely your transmission will be subjected to -40C generally speaking flow characteristic is more important to hydraulics such as an automatic transmission than. It is also my opinion that the Valvoline is a better product with a better additive package of friction modifiers, anti foaming & wear agents. In any case either will meet the requirements necessary so it pretty much comes down to personal preference. Hope this helps.

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  • "Although it is unlikely your transmission will be subjected to -40C" cries in the Canadian prairies.
    – Scorb
    Commented May 23, 2023 at 14:40
  • What do the brookfield viscosity numbers specifically mean? You mentioned they are torque required to a turn a gear....doesn't that mean that a high number means more torque was required, therefore the Beck Arnley took more torque rather than less?
    – Scorb
    Commented May 24, 2023 at 4:46
  • My apologies, I had a dyslexic moment and reversed the numbers by accident. You are correct. I don't have any experience using the BeckArnley but have used the Valvoline professionally in thousands of vehicles with no issues to date.
    – ShadowHemi
    Commented May 30, 2023 at 23:38

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