But I would bet it uses the same base as their other 90W140 gear oils. I am only comparing within their product line.

And don't forget that viscosity <> coefficient of friction or internal friction of the liquid.

So my thought is that the 90W140NS may be the best choice as it is GL5 without the modifiers and without the solids.

I think I'll give the tech line a call and see what they say.

Redline will say 75w90NS, thank you call again. They've done so on i-club and nasioc numerous times. Because Shockproof is GL4, they will not recommend it, even "unofficially", for GL5-required uses. So if nobody else had done it, I would not use it in my Subaru. However, lots of people had, so I tried it, and like it. For a daily driver, I will use 75w90NS after first change interval, and for track/autocross cars I will trade the longer life of the R&P and to a lesses extent bearings and gear surfaces for better protection against bad shifts.

Lightweight Shockproof in my STi tranny has been nothing but awesome to me.