This post was a reply I made in the transmission forum, but since it covered most all fluids in the car I felt it was worth sticking here. Please post your additional comments and recommendations, but post a WHY with your WHAT. One-liners like "I use Marvel Mystery Oil in my gearbox" will be removed, as well as get you laughed at. Keep it on-topic and informative. This isn't a poll.


Here's my personal fluid recommendations. They aren't the only solution, others will post different choices, but there is no "one" right answer in this case.

Engine oil: 5w30 full synthetic in colder climates. I usually use Mobil 1, but any full synthetic will do. Other favorites are Redline, Neo, Motul, Royal Purple, etc.

Gearbox: Synchro care is critical. There is much debate about what is good for the gearbox. I recently had 75w90 Royal Purple added when I changed my clutch and flywheel, and I don't think it's good for our gearboxes. This is because it is too slick- the synchros can't grab and this helps cause grinding. I will be switching to 75w90NS Redline or Neo 75w90 RHD when I install my 6MT, as these have what are called "friction modifiers" which help the synchros to work faster. Others will say to simply use the factory fluid, but nobody seems to have a certain grasp on exactly what it is or where to get it. Most dealers simply use plain 75w90 dino gear oil for their changes. Yet another option (bet you thought you were just gonna get some real quick answers for you to run to the auto parts store with, huh? ) is to try a 'cocktail' of different oils, usually including dino oil or what is known as GM Synchromesh. Before you settle on what to get, do a lot of searching and reading and decide on what you want to do for your own reasons. Gear oil is a compromise- choose according to what you think is most important. Again, I will be going with a synthetic 75w90 with limited slip additive. Your RS has a limited slip rear diff, so use the same stuff you put in the gearbox.

Coolant: Use whatever you want, as long as you flush out the old stuff entirely. Rather than running a 50/50 mix, try going with about 65% water/ 35% coolant. The coolant is there to not freeze and not boil- water is the best heat exchanger. I also recommend using a product like Redline Water Wetter. The car will run cooler with no other mods.

Brake fluid: I use ATE SuperBlue Racing, but like engine oil, any full synthetic DOT 4 or DOT 5.1 fluid from a name brand will do. Everyone has their favorite. Avoid DOT 5 fluids if you like to drive hard.

Clutch: Same as brake fluid. Shouldn't need to be changed, but bleeding it wouldn't hurt. Bleeding it, however, will require a refill so it might as well be flushed.

Boost fluid: Just kidding.

Cheers,
Kevin

 

Thanks for the excellent post. However, I'd like to add the following:

Be VERY CAREFUL what engine coolant you use -- it must be completely safe for ALUMINUM ENGINES specifically. There are a few low-cost brands out there (Peak, for one) that contain silicates which can degrade an aluminum radiator and even cause engine block wear. Both my dealer and their lead shop mechanic said WRX models in particular should use the factory Subaru fluid (which they sell in gallong jugs as well) and nothing else, because of this danger. Normally I would be skeptical, but the mechanic pointed out that they could actually make more money selling a brand-name fluid to customers -- they would just rather take the loss and avoid replacing radiators under warranty

For the record, I run factory subaru coolant (which is premixed 50/50 with water) + 1 jug of distilled ionized water + 1 bottle of Redline Water Wetter.

 

Good info meilers, thanks!

 

i prefer Motul RBF 600 for brake fluid. it's DOT4, so it doesn't requre the attention that the DOT5 silicon based fluids do. it has a higher boiling point (if i recall correctly) than the ATE super blue. i've always run RBF600 in my bikes and now in my car, i've got nothing but good things to say about it -- though brake fluid is toxic stuff.

i'm getting ready to change the gearbox oil, and i'm either going to go with Redline or Motul. both companies put out quality products, and as long as you get the correct weight, i doubt you'd be able to tell the differance between the two.

as for coolant, water, aluminum safe coolant and water wetter. some of the other water wetter type products work from what i've read/heard, but i've only personally used water wetter. it works for me, and i've got no complaints.

 

If it's the same color as what you had, it's fine. If it's a different color, than flush it, run on just water for a couple days (unless it will drop below freezing), then drain and refill it.

 

Cobb Tuning is recommending
Redline MT-90 for the transmission and
75W90NS for the rear diff.

Apparently the problem people are having is that what you choose for the transmission has to do two things. Have enough friction to protect your synchros AND be slippery enough to protect your bearings.

MT-90 is only a GL-4 but it should work best for protecting the synchros. I do so much stop and go driving that I'm more worried about that at the moment. If I end up doing a lot more long drives with little shifting I'll probably switch my transmission over to something slipperier like the Redline 75W90 (not NS)...

 

Could this be the main cause of this headgasket problem that people keep talking about?
Subaru (from what I read on NASIOC) has placed out a safety bulletin for headgasket replacement and highly recommend the use of Subaru fluid only.

 

That has to do with leaks eventually developing from thermal expansion/contraction.

 

If you have the dealership do your services, they will add it for free, but they won't give away the conditioner for free. It's inexpensive, however.

 

Wrong. Redline 75w90NS does NOT have a friction modifier additive.

Friction modifiers are additives that increase the "slipperyness" of the oil, and are reccomended for useage in limited-slip differential applications.

 

Correct. I swear, NOTHING seems harder for me to learn and retain than the differences between gear oils. As you pointed out, friction modifiers are for clutch-type LSDs that are too aggressive and need to be smoothed out. Our LSDs do need the higher friction properties of 75w90NS if you were to go with Redline however.

 

Yeah, I'm in the same boat as you, lol.