Frankenmix differential oil 2008 Subaru 2.5 n.a.

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There's already 37,000 miles on the original fluid, and Subaru says to drain and refill IF it is on "severe duty", so I decided to drain and refill front and rear diff, having already drained and refilled x 4 the ATF with Valvoline synthetic ATF.
WalMart had ST gear oil on sale, but it was 75w140 synthetic, which is too heavy, 75w90 - which is the right weight, but only semi-synthetic. so i bought 4 q. of 140 synth, 3 q. of 90 semi synth(which was all they had) and FRANKENMIXED IT!!! with 3q. of Valvoline synthetic ATF, to thin it out to a 70w90 in order to get a fluid that should be 80% synthetic. Result:extremely smooth, easy rolling.
 
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I looked up the viscosity figures. valvoline is 30.3 at 40 degrees and 6.3 at 100 degrees, CST. this makes it too thin to be used in a differential, but perfect for thinning down a 75w140. and since it is a gear oil, it probably has the high pressure additives that are needed. the final mix is 30%ATF, 70% "gear oil", so the EP additives are not too heavily diluted, and since it is only an NA engine, it just cannot put out the extreme torque that a Turbo wrx sti version would, which is specced for the same oil, for that reason i thought a thinner oil would work better and run with less friction.
 
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