So an oil company actually *recommended* mixing...

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I was talking to RLI about oils for my differential. The factory fluid was labeled 75w-90 when it came out, but on the new scale it's a 75w-110. Very rare grade. Most people run either a really thick 75w-90 or, per BMW's recommendation, a 75w-140. I'm pretty sure the latter is just because BMW only wants to stock one kind of fluid at the dealerships. I have run Red Line 75w-90 and am now running RLI 75w-90 LS, and both seemed to work very well. This time, though, I wanted to try to get as close as possible to the original spec. My diff is getting old (152k miles), and the clutches are wearing out; at a minimum, I probably don't need as much limited slip additive, and it wouldn't hurt to quiet things down a little with a thicker lube (these things area always noisy). I called up RLI for some advice, initially intending to ask for a custom blend. However, they said I could just mix equal parts of their 75w-90 LS and 75w-140. The 140 doesn't have FM, but they said the 90 probably has more than I need anyway; if the clutch packs start chattering, I could add more FM as needed. Apparently those two gear oils are almost identically formulated, so it makes sense that they should mix. If they were to custom formulate something for me, that's essentially what it would be anyway. I just thought it was interesting to see a company actually recommend mixing. And before someone asks: - No, Motul doesn't have a rear end lube that I'm interested in (yet). - I know Amsoil has a 75w-110. I am a big fan of their gear lubes, and I am not going to say RLI is better.
 
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I think it's pretty typical for the same manufacturer to use the same add pack in their various gear oils (of same type). Only the Vis of the base oil will be different.
 

MolaKule

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What Tempest said. The viscosity for a 75W90 is around 15 cSt and the viscosity for the 75W140 is 26 cSt. The average value for these numbers using a 50/50 mix is 20.7. The viscosity value for a 75W110 alone is 20.5 cSt, so yes RLI is correct.
 
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