which tranny fluid to use

You should use any GL-5 rated fluid. I would use conventional fluid, but that's just me.

The NS is designed to be used in clutch type limited slip differentials so it has "friction modifiers" which make it LESS slippery - do NOT use it in the transmission, the syncros will not work properly. It is fine for the diff.

I apologize in advance if this post is seen as "trolling"

You should use any GL-5 rated fluid. I would use conventional fluid, but that's just me.

 

The NS is designed to be used in clutch type limited slip differentials so it has "friction modifiers" which make it LESS slippery - do NOT use it in the transmission, the syncros will not work properly. It is fine for the diff.

 

 

 

 

I apologize in advance if this post is seen as "trolling"

Not to be a stick in the mud, but you have it backwards. The NS is the one approved for manual transmissions.

From the Redline website:

75W90

"This product is not designed for use in most manual transmissions or transaxles in passenger vehicles, since the extreme slipperiness may cause synchronizer mesh issues that lead to shifting problems"

75W90NS

"Designed for transmissions and transaxles - helps to slow synchros for easier shifting across a broad temperature range"

Crap

Actually I had it half-backwards.

I apologize in advance if this post is considered "trolling"

Friction modifiers operate exactly as Rao said, and their addition is a double edged sword. They are used to accomidate limited slip differentials, but also to handle the extreme pressures of high-offset hypoid gearsets (which is exactly what the front diff of a Subaru is). It's not really a solution I like, because it robs Peter to pay Paul. You give up synchronizer efficiency to help mitigate loads in the front diff. You have an adequately protected front diff, but then get the "normal" clunky Subaru shifter. Anyone who thinks a clunky shifter is "normal" in a Subaru, is typically using a fluid with some kind of friction modifier. This behavior is not normal, and there are better options out there.

I know from personal experience that Redline's 75W90NS is absolutely RUBBISH in a Subaru manual transmission.

As I mentioned in the exact same thread posted somewhere else on the board, I've had excellent luck with the Motul Gear300 (75W90) and Redline Lightweight Shockproof combination.

Motul's gear300 is a non-modified fluid and I only use it because I think they make a good fluid. Any super high quality GL-5 compatible synthetic fluid is a fine substitute. Extra-S, Amsoil, Mobil-1, whatever, are all probably fine.

Shockproof has some really neat properties. Namely, an ultra low internal friction with an ultra high film thickness. What that gives you is a fluid with the viscosity of a very thick gear oil (I think the lightweight has a viscosity similar to a 75W140), but the friction of a thin motor oil. Both of those features mean an awesome load handling capability.

I've described elsewhere in great detail how a synchronizer actually works, and why Shockproof is an amazing fluid for the job. The synthetic 75W90 is merely to help maintain consistent flow during varying temperatures. You COULD run 100% shockproof, and I did in my 176,000 mile old Forester transmission. It got a little notchy when cold, but shifted like a brand new transmission when warm.

Good fluid costs money, but in the grand scheme of things is $60 every 15-30k really a major headache?

I change my trans and rear diff fluids every 15k because it's available to me and it doesn't cost me retail price.

I wouldn't use Redline's 75W90. I'd use a higher end fluid. Motul, Mobil1, Extra-S, Amsoil, one of the "premium" brands is typically safer. I use Motul because it's high quality, and passes European standards and is individually approved by various makes to be used in their high performance models.

Just use Motul or other in the rear diff.

Mobil 1 is "higher end" than Redline

I only use parts that pass stringent European standards, which are a lot better than any other standards because they are European.

When have Europeans gotten things wrong? Especially since Germans and Italians endorse it! What could go wrong?

I've just had not-so-good experiences with Redline's traditional gear oils. Their 75W90NS is rubbish, and I wouldn't hold out much hope for their non-NS formula. Shockproof is really the only fluid they make that I like. The same goes for Royal Purple. I've just had bad experiences with it.

Motul is available to me, which is also another reason I use it. I likely wouldn't go too far out of my way, but it's good fluid overall. If I had to pick something off the shelf at Autozone, I'd probably go with Mobil1.

Not everyone is lucky enough to be able to run ATF. And I actually mean that, which is odd.

The problem is that you can buy Redline and Royal Purple at your local auto parts store, which everybody knows makes them no good.

I would only use ATF that meets European Standards, and I also change the fluid every time I drive the car, but that's just me. If others don't car as much about their cars that is their problem.

In my Legacy I am using plain jane dino GL-5 rated 75w90 gear oil and do you know what? It works

You might be onto something there. If you don't have to mail order it, it's not worth having.

I thought you'd use ATF that meets Ugandan Standards. Their climate is much harsher, and that closely simulates the loads a supercharged V8 puts on a transmission. If it can meet Ugandan Standards, it's good enough.

Have you considered changing the fluid before AND after driving the car? It's called a placeholder interval, and it's critical to the longevity of the transmission. The post-operation flush keeps contaminants from settling on hot parts, and ensures that all wear materials are properly flushed away. The fluid that sits in the transmission overnight, the placeholder fluid, keeps things wet which means it's better. Replacing that fluid before driving the car again flushes out any residual contaminants and provides you with a clean-slate-start. Only the people who really care about their car follow this procedure.

As a side note, how does this always happen?