GL-5 Gear Oil for FWD Manual Transaxle?

[carlito]

Hi all, I know this gear oil subject has been discussed extensively, However I am unable to locate any thread specifically discussing FWD manual trannys. I want to change oil on my1989 JDM legacy which has a manual FWD transmission. The owners manual calls for GL-5 oil for both the FWD and AWD, but reading the threads here I get the impression that GL-5 is bad for the soft metals in the gear box. My gears are notchy but they shift fine, the reverse gear sometimes grinds if I shift before car comes to a complete stop. I do not know which oil is in the gear box currently, Could someone clarify whether its safe to use a GL-5 oil such as one without ''limited slip additives'' in a FWD transaxle or should I go with a GL-4 oil?

[vrg3]

There is no FWD versus AWD issue here. The GL-5 requirement for Subaru transmissions has to do with the hypoid gears in the front differential.

I wrote a pretty thorough explanation here: http://bbs.legacycentral.org/viewtopic. ... 15#p141872

At the end of that writeup I mention that I chose to use GL-4. Since then, that transmission blew up. It was a snapped gear in the gearbox, not the differential, but that was after lots of drag racing, so I don't think we can use that as a data point. When I put the new one in, though, I decided to instead use Redline ShockProof, which isn't exactly GL-5 but is similar.

But in short: GL-5 can indeed be hard on the soft metals that make up your synchronizers, but GL-4 is the wrong oil for the hypoid gears that make up your differential. All of us are wrestling with the same issue to some degree. Since you ask if it's "safe" to use GL-5, I have to say that it is absolutely safer to use GL-5 than GL-4. Subaru spec'd it that way for a reason -- even if your synchros wear out, you won't be left stranded anywhere. Not so if your diff grenades.

Also -- Don't bother with "limited slip additives" or "friction modifier." That stuff makes sheets of fluid behave more homogeneously when under the pressure and heat of a clutch. Unless you have a clutch-type limited-slip differential, it's not relevant.

Also also -- All of this has nothing to do with reverse. Reverse has no synchronizers. Never try to shift into reverse if the car is not completely stopped. And if it hasn't been stopped for more than several seconds, first shift into a forward gear and then into reverse.

[carlito]

Thanks vikash that was very helpful. Now the remaining challenge is to find a good quality GL-5 with no friction modifiers. Down here the most readily available oils are repackaged and sold in gas stations under their brand names as well as locally produced oils with very little infomation on the bottle showing certification specs. I hate shooting in the dark when it comes to my car so I prefer well tested products. So I have zeroed in on Mobil1 synthetic 75w90 castrol syntec 75w90, valvoline synpower,75w90 valvoline durablend 80w90 and Mobil delvac these are locally available however from research it seems like all the above except delvac have friction modifiers. I think I will go for it. Which other good quality GL-5 oil is free of modifiers?

[vrg3]

Delvac looks like pretty good stuff. Redline 75W90NS is another option. It supposedly is formulated specifically to meet GL-5 requirements but also work well with synchronizers. I used it for a while and it worked pretty well in my experience. It made for easier shifts than ShockProof but not quite as easy as MT-90. You'd probably have to mail order it, though.

[Legacy777]

From my experience, I've found the syncros in these transmissions don't like synthetic fluid. It's too slippy and as such can't slow down as quickly to match revs between gears. Honestly, a regular non-synthetic GL-5 oil is going to be fine.

[carlito]

Thanks guys, I cant find Delvac I think I will just settle with valvoline conventional 75w90 as its supposedly closest to the OEM gear oil or valvoline 80w90 despite the LS additives. For engine I chose Valvoline vr1 10w30 conventional. I also cant find oem subaru coolant. We have Abro radiator coolant which subaru dealers seem to prefer as well as zerex products. Have read warnings about dexcool and other OAT/HOAT coolants. Will zerex original green coolant be a good substitute? Its ethylene glycol based but contains low levels of silicates. Its a newly swapped engine so I want to do everything right. I need to flush thoroughly and put in the best OEM coolant substitute.

[Legacy777]

I use regular prestone green antifreeze. I think the main issue with the additives and silicate levels comes into play with some of the newer engines & head gaskets. I've never heard of any issues with the older Legacies.

[vrg3]

I believe Subaru specifies any ethylene glycol coolant that doesn't contain any silicates or phosphates. If I remember correctly, the standard Advance Auto Parts brand coolant meets these requirements.

Silicates and phosphates are actually supposed to be protective for aluminum engines. The issue is that after a while they precipitate out of solution and then clog or grind. You'd probably be okay using Zerex as long as you were diligent about changing the coolant regularly.