Hmm, interesting. I guess I should get myself more up-to-date. Noise and vibration suggest the oil is shared (like Eaton design). Emphasis on “suggest.” If so, that noise and vibration could be due to friction modifiers, which are near ubiquitous in 75w90’s nowadays. (Lots of threads on this, not just my opinion)…in fact I have no decent opinion, though will avoid FM’s if/when I install a Torsen in my camper van. Those modifiers apparently are to allow usage in clutch-type LSD’s. Your Redline, some Motul’s do not have the FM’s, so might be exempt from the warning. In other words, for emphasis, my interpretation is, it’s not the viscosity that would be a problem, it’s the modifiers a Torsen doesn’t like. FWIW, some Redlines, some Motul’s, and Mobil Delvac do not have the modifiers. Amsoil and HPL do, but some installers / reps claim theirs don’t cause Torsen issues (not pointing at Pablo). Eaton says to stick with mineral-based, without modifiers. GLWT. I have no clue. But FWIW, Motul Gear 300 is “closer to” a 75w85 in thickness than most 75w90’s, has no modifiers, and is a VERY high quality fluid. Subaru (Idemitsu) fluids are also quality, so that might be another safe alternative. That said, I don’t know that their 75w90 is free of “bad”-modifiers. But if they still suggest same fluid for a Subaru stick shift as a rear differential, my “guess” is it is OK for both, as those modifiers don’t play nice in a manual trans either. One clue might be if it is GL4, GL5, or GL4/5. Subaru recommended GL5, but as stated above, many GL5 fluids added friction modifiers. but I am not an expert on new Subarus (maybe not even the older ones), but I think I’ve read they have a different fluid recommendation now for transmissions a differentials…for the few manuals they recently made. Back in the day, they offered an “Extra-S” fluid that worked well enough in both stick-shift front transaxle and rear differential. IE no modifiers. That stuff is gone, and I don’t know how good its replacement is, though replacement was compared to several other high-end differential fluids and performed well…in generic differential fluid testing (not transaxle, not Torsen, not Subaru).
back in the day, there were 2 Redline 75w-90’s, one with modifiers, one without. The internet gurus of the day didn’t like either in the standard transmissions. In a differential with a sealed Torsen unit, it didn’t really matter as much. Times may have changed…from Subaru’s perspective, and from Redline’s. Don’t drain it, but I would consider the Motul Gear 300 next time…or get on the BRZ website and see what those yayhoo’s think. Enough of them are bashing the heck out of their boy-racer sleds, and probably have better insight than me.
