see a lot of people asking if their specification 80W-90 can be substituted with something else. Invariably the chorus here says "use 75W-90 is better because muah, synthetic".
Since my car differential and transfer case also requires 80W-90, I kind of thought... why Toyota din't say 75W-90 if that was so great?
To start, I measured my differential temperature after running 30 minutes in 70 F ambient. Result: 110...117F. For people in the rest of the world that's 43...47C.
I am curious... let's take a look at the viscosities of various gear fluids at this temperature. Not at frozen -50C, not at scorching 100C , but at normal temperature for this application, 40C.
Well... looking at that table I don't think at all that 75W-90 is an adequate substitute for 80W-90.
Personally I went on the Redline 75W-110 route, because that's a synthetic with 40C viscosity closer of what my car manual specifies.
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