Hurting 5MT. 75/90 = no. SAE 110???

[rao]

[Lots of gear oil info deleted for brevity]

 

Let me see if I understand this - you are suggesting that using the manufacturer's recommended type of oil is the right thing to do

 

This place has really gone downhill

 

The question was answered in the first response:

 

I wouldn't.

[BAC5.2]

I'm disappointed you didn't quote full bore.

 

If you aren't going to use shockproof, use extra-s.

[ehsnils]

OK, thread went out of line so it's now closed, I don't think that there will be any good answers here.

[SBT]

This has been covered before, to death.

 

I won't give a lecture on the fundamentals of fluid mechanics, nor do I have any desire to summarize almost a decade worth of intimate experience in the Subaru 5MT in a few short lines. Especially since I've done it before in other threads.

 

But here is a cliffs notes version, so you'll stop whining.

 

Two factors determine the suitability of a fluid for use in a transmission. Film thickness and viscosity. The two are mutually independent. You can have high viscosity and low film thickness (something like honey is a good example), and you can have low viscosity and high film thickness (something like pudding, or mayonaise). These are the most critical factors in transmission operation.

 

If you think of two plates seperated by some small distance, with fluid between, and you slide one plate (in the same plane, so the distance between the plates remains constant), you have some shear action going on. High viscosity is a resistance to shear, and it is difficult to slide the top plate relative to a lower viscosity fluid. As the distance between the plates increases, you will soon exceed the thickness of the fluid, and there will be no shear resistance at all.

 

Bearings, synchros, and gears all need different things. Bearings need lubrication more than anything. Since bearing surfaces are rolling, and the bearings in a transmission don't actually see massive loads (because everything is axially aligned), a low viscosity oil is ideal. The low-loads is the critical factor here. Something with good lubricating properties, and a low enough viscosity to freely allow the bearings to roll without slowing them down.

 

Straight 110 can't really provide that. It's viscosity is relatively high, and that would cause bearing drag.

 

Extra S is good, because it's nothing more than a super high quality 75W90, probably the highest quality 75W90 that you can reasonably purchase.

 

Shockproof is good becasue while it has a very high film thickness, at operating temperature it has a very reasonable viscosity. At 100ºC, Shockproof has the viscosity of a 30-wt motor oil (which is almost a factor of 2 less than your 110), but the film thickness of a heavy-weight gear oil (something in the 75W140 range). This second bit plays a role later.

 

In terms of bearing performance, Extra-S is probably the winner with Shockproof a close second.

 

Gears need film thickness. At high loads, viscosity matters less than film thickness. This is because, as surfaces begin to close in, the pressure between them becomes immense. High film thickness gradually dissipates the load between the mating surfaces. Kind of like if you were to roll a bowling ball over a marshmallow, you'd have a pressure gradient far less dramatic than if you rolled a bowling ball over a jelly bean. In deforming the thick fluid, you dissipate energy that would have normally gone into the other surface.

 

This is where GL-5 ratings come in. Hypoid gearsets like our front differentials, impose ENORMOUS loads. Thousands of pounds of force between the teeth. A thin oil is going to do little to nothing to cushion the interface between the gears. There will still be lubrication on the gears, which means they won't over heat and friction between the faces will be low. But there is nothing to absorb and dissipate the energy between the two surfaces. GL-4 fluids have less complex microstructures with weaker molecular bonds, and aren't suitable to those types of loads. They merely break down, and you'd have metal-on-metal contact. In fact, that last point is exactly what happens if you use Uncle Scottys mix. I've seen cars running GL4 fluids completely chew through the ring and pinion in a few miles on the track. In street cars, it takes longer, but still happens. I'll never forget one transmission. We pulled it apart, and I cut my finger on the pinion, the teeth had been ground to a knifes edge. I remember almost every detail about that car.

 

Extra-S is, again, an extremely high quality GL-5. It can handle this added load without much problem, though with a low film thickness, it doesn't give you the protection of something thicker, it does the job more than adequately.

 

Shockproof DOES have the film thickness and molecular structure to withstand these loads.

 

If Shockproof was a marshmallow, Extra-S would be a JujyFruit and a GL-4 would be a rock or something. Varying levels of impact mitigation. For all intents and purposes, all you really need are jujyfruits. When you start making power exceeding 600ft-lbs, and begin drag racing, that's when you start to need marshmallows. Dogboxes, if you want to actually drive them daily, will need marshmallows to cushion the engagement teeth. You don't HAVE to have marshmallows, but it makes the whole proces a lot less violent. I daily drove a dogbox for 6 months, and it was a dream. I actually would not be opposed to having one in my Outback, they are that much fun to drive.

 

Gears also need low viscosity, to prevent overheating of the surfaces and decrease friction between the surfaces. Your straight 110 will protect the gears well, but will induce an awful lot of additional drag.

 

When it comes to gear protection, there is no contest that Shockproof is the winner here with 110 coming in behind it, and Extra-S taking up the rear. Film thickness trumps all when it comes to protecting impact surfaces.

 

But in terms of gear efficiency, the order is somewhat reversed. Shockproof takes the lead because it has both the thermal mass to absorb heat, and the low viscosity to decrease friction. Extra-S comes in second, because it's a well rounded fluid that can handle the loads generated by a gearset, but has the right viscosity to reduce friction. The 110 tails in a big way, and you can measure an increase in surface temperature when using a fluid like that (I know, because I've actually done research regarding this kind of thing).

 

Now we have Synchros. Synchros require viscosity to work. There's not much around it. There is a finite limit, and that is design dependent. For the most part, you have these two sliding plates. Too low viscosity (something like baby oil or something), and the fluid will shear, you'll have metal on metal contact, and you'll force the collar over the balk, and the mating gear will still be spinning (you get grinding). Too high viscosity, and the synchro will constantly "drag" as if they were always partially engaged (since there is some level of preload on the synchros).

 

Guess where your 110 fits?

 

This is the main deliniation between Extra-S and Shockproof is here. Extra-S is, again, nothing more than an extremely high quality 75W90. It still has the thickness and viscous properties of a 75W90, and that puts it at a mild disadvantage. With a higher film thickness, you again get the marshmallow effect. Thicker film means you begin the process of synchro operation sooner, as you push the surfaces together, shear stress increases.

 

Remember the plate example? A synchro is the same thing. But NOW, unlike before, we are trying to MOVE the bottom plate. So if you think about the functionality of the fluid, the fluid is transferring energy between one rotating surface to another. The whole goal of the fluid is to bring one shaft up to the same speed as the other, which will then reduce the relative angular velocity between the plates to zero and thus eliminate any shear stress in the fluid. It's all about equilibrium.

 

A thicker fluid will begin this process with the plates further apart, and as they get closer will resist "squishing out" better than a thinner fluid. That is the number one reason why Shockproof is so well regarded in it's ability to stop transmission grinding.

 

The ideal fluid for a Subaru 5MT is, without any doubt, a low viscosity, high thickness fluid. Extra-S is a perfect choice, because it is the best of the 75W90's. Shockproof is my choice, because it wholly embodies the low viscosity, high thickness concept. SAE110 is high viscosity, and high thickness, and is wrong for almost every component of transmission operation.

 

And I bet you'd see a 10% decrease in economy using the SAE110.

 

That all said, I continiously recommend using 3 quarts of Lightweight Shockproof and a quart of Motul Gear300 75W90 (or, actually, a quart of Extra-S would probably work really well). In all my years in the industry, I've never met anyone who has used this mix and been disappointed. If they were, it was because something mechanical was actually broken.

 

If you don't want to mix, for some reason, then straight Extra-S will be your next best option.

 

Using a straight weight 110 might solve your grinding, but it'll feel like your towing an extra 350 pounds and your fuel economy will drop like a rock.

 

In closing. Use 110 if you want. But when you ask for my opinion, it was given in the very first reply to this thread. I wouldn't.

 

 

There are a few disclaimers I must make. First, this analysis applies to Subaru 5-speed transmissions only. Some others are similar, but none are exactly the same. Some manual transmissions can use motor oil and be perfectly fine. Obviously, some can use GL-4 fluids. Some require very very thick fluid, like straight 110. Some need even lighter fluids than ours. While the concepts of requirements are the same across all transmissions, the application of those requirements can be very different.

 

Nicely restated (what for the fifth or sixth time on the board?). Hopefully our new member will get some use out of it.