What’s your favorite flavor of GL-4 gear lube?

The transmission in my sons SX4 failed this week and we're going to be replacing it with a used trans. 

Until we open it up I won't know what let go. It was noisey when accelerating or decelerating, progressed to not always staying in first gear unless the shifter was held, and now currently it rolls in Neutral, but once it's in gear it will bind and stall out the engine when the clutch is released.  The shifter feels like trying to stir a 5 gallon bucket of drywall compound  

We had replaced the lube with Synchromesh last year since we couldn't find a true GL4 rated fluid last year that wasn't also GL5. My understanding is that GL5 is not good for synchros and yellow metal parts. 

The failure has me second guessing the synchromesh choice last year and now looking for a GL4 rated fluid to use in the replacement trans. 

Is there any GL4 rated lube available that aren't also GL5? Or is my concern about the GL5 unwarranted and just go with a good GL4/5 rated gear lube?

I am running Redline 75w90 GL-4 in my Suzuki SX4's six-speed trans.  Helped the shift quality a little; these boxes are notchy on the 1st/2nd shift, especially bad when cold, and generally not that great to use.

Discussions in the classic car community say that the damage GL5 can do to yellow metal is so minimal, you will be long dead before it's an issue.

I use Redline Heavy Shockproof in all my own stuff with no problems.

We use Lucas non-synthetic 80W90 at work, the synthetic leaks out of older transmissions easier.

I don't think GL5 is a destructive problem for the sychros, it's more of a "We are too slippery for the blocker rings to bite properly" situation.  I've delayed a few transmission replacements in my time by putting thinner oil in to improve shifting.  ATF, even.  Not really suitable for heavy towing, but for commuting, no problem.
Thats been my experience, anyway.

This was a big subject on the SX4-specific forums at one point (those forums seem fairly dead these days) and I think I remember people claiming trans failure after GL5 fluid was put in their transmissions.  

Honestly it's not hard to get GL4 fluid if you can order it and wait a little bit so why not go for it?  

Redline MT-90 worked well for me in a gearbox that specified gl4 and was crunchy with a GL5 

ShawnG said:

Discussions in the classic car community say that the damage GL5 can do to yellow metal is so minimal, you will be long dead before it's an issue.

I use Redline Heavy Shockproof in all my own stuff with no problems.

We use Lucas non-synthetic 80W90 at work, the synthetic leaks out of older transmissions easier.

I had to replace a Land Rover transmission because of yellow metal damage. Took a bit of looking to find fluid with the correct GL rating for an old 60's LR box. NAPA to the rescue, by the bucket. Basically, tractor lube. Which is appropriate.

In reply to Keith Tanner :

NAPA was the only place locally that that had GL4 when I changed oil for the first time in a 449,000 mile 4RUNNER box.

I'm a fan of Redline and they offer a pretty wide selection of weights along with some heavier GL-5 fluid without any of the friction modifiers that cause the slippery issues that prevents synchro's from working properly.

Redline Transmission Oils

GL-4 flavours

MT-LV (70W-75)

MTL (75W-80)

MT-85 (75W-85)

MT-90 (75W-90)

GL-5 Non slippery (ns) flavours

75W-90ns 

75W-140ns (recommended for older transmissions to quiet down noisy bearings etc)

My experience in my 400,000 km Getrag in my E28 is that it is happy with all of these but the lighter ones shift better when cold. Redline even recommends mixing the 75W-140ns in with one of the lighter fluids to come up with an in between weight if that is desired. 

Thank you everyone for the advice. I've added the MT-90 to my Amazon cart and will be pulling the trigger later today.  

Im waiting for my wallet to catch its breath after buying the replacement trans and ordering a clutch set and flywheel from RA. 

I also need to find a replacement cross member that runs front to rear under the engine to repair where it rusted through. Seems to be a common rust area, so used ones are costing a premium and new are few and far between. 

This is not the flavored lube discussion that I was hoping for. 

In my Suzuki days synchro was the go-to for their lousy transmissions, and it was widely used with success so I don’t think that was your problem. 

I bought so many of them with the bad synchros, and crunchy 1-2 shift that I experimented with just about every oil I could find to either fix it or keep it from happening. When you found a good 4.39-1 trans you didn’t want it to go bad. Synchromesh was good, ATF was good, as was synthetic screw compressor oil (I was servicing compressors and vacuum pumps at the time) but the best was an ISO 100 vac pump oil. It was like magic in an old trans, and would fix that 1-2 crunch and notchy shift almost every time 

I will say I've had very bad experiences with GL-5 and synchros.  I've actually had much better luck running ATF in my manual when GL-4 was unavailable.