MT synchros and lube
 

I can say that in my case, no is the answer to both questions. However the proper Redline gear lube for your transmission is MT85. I have it on the shelf waiting to go into mine.

Well, my owners manual says 75w85 or 75w90 and I was going to go with the heavier oil based on the better shifting at lower temperatures. I hate to waste the oil if it won't help. I'm in Clute. I've got a 3 month, 3000 mi warranty from Texas Direct. Damn, they've got a lot of nice cars. Thinking about it the temperature thing is totally bizarre because even in the cooler weather the gearbox shifted well after 50 miles of driving.

Redline MT-90 GL4 in mine for 2500 miles, no new problems (My 5th gear synchro is almost dead (70k)). If a MT is designed for GL4, you MUST be sure the proper gear oil is going in there.
A big, big issue in Evo world.. GL5 will trash those transmissions real quick. I cant speak from experience with the 6mt in these cars, but I wouldn't dare use anything other than GL4 gear lube.

Hmm... A little digging produced this thread:
MT-90 from Redline - Nissan 370Z Forum
It seems like a good breakdown of various behaviors of different fluids. Maybe our transmissions are more finicky than I realized.

Ryan@RJM is someone who's experience and opinion I would trust. I may try the MT90/MTL/Amsoil Synchromesh cocktail. Or since I already have it, 2 parts MT85 and one part Amsoil Synthetic Sychromesh since I already have the MT85 and ti's the same thing as the MTL/MT90 mix.

Based on your respone it doesn't sound like these gearboxes are exactly rock crushers. The Redline MT90 definitely says it's GL4.
http://www.amazon.com/Red-Line-50304-Transmission-Transaxle/dp/B000CPCBEQ/ref=sr_1_2?ie=UTF8&qid=1397060201&sr=8-2&keywords=redline+oil<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

I'm looking in Amazon and don't see anything called MTL. Is that the Manual Transmission and Transaxle lubricant? BTW, is there someplace to buy Redline in Brazosport? I remember looking on their website once upon a time and it showed a place in Sweeny.

I never find anything locally, I always get it online.
Red Line Synthetic Oil - Gear Oil for Manual Transmissions

I'll x2 my experience with various Redline fluids in Evo 8/9 gearboxes. Word for word I experienced what Ryan wrote up. My personal opinion - MT90 works for me.. I simply don't expect the transmission to shift smoothly until temps get up. Being a believer in operating temperatures I always prefer the heaviest oil I can safely use in whatever application, as I never expect or attempt "cold performance". Factory recommendations apply to the general public, that's something to keep in mind. I used to run Redline Shockproof heavy in my 500+whp Evo's transfer case and rear diff. That stuff is rated 90w250! And I never had an issue driving on the street or a failure launching at 8000rpm on the track ;)

Just to clarify, I don't have much 370z/G37 transmission experience, I have no idea, first hand, how much abuse the 6mt in these cars can take. I just got into these things 4-5 months ago. I have ~11 years of experience owning/building (personally and professionally) Evo 8/9 as well as Subaru Wrx/STI.

The point I was making is -- the most important part of the label on your gear oil is ** GL4 **

Gear oil that says something like "GL4/GL5 compatible" I stay away from. GL5 has friction modifiers in it to make it more "slippery". Synchro's designed for GL4 oil, will not work right with GL5 oil and WILL cause damage to the transmission. Saw this hundreds of times in the Evo community.
Vice-versa for a transmission designed for GL5 oil.

I've got GL4 75w90 (as it turns out) name brand oil in it. Pennzoil synthetic. May not be the optimum stuff but the synchronizers shouldn't grind. It's going to the doctor.

OP,

Based on this forum and the 370Z forum, I would say the 5th and 6th gear synchros are weak. Some people will tell you not to skip shift this transmission. I do skip shift, but I pause slightly to let the rpm drop, which acts like double clutching.

Greg

Try the Redline out. Change it again after ~5000 miles, flush all that other stuff out. Also be sure your clutch is working properly. The gears are obviously more prone to grinds if there is a potential clutch release problem. (search clutch CSC failure)

x2

Running Red Line MT-85 approx 12k mi. with no issues besides the 2nd&5th syncro via quick shifts.

I hope by "lube stop" you dont mean one of those generic drive in, drive out quick lube places?!? because those places and "screw it up" go hand in hand. I wouldn't expect anyone working there for minimum wage to give two doodly poos about any one elses car. Let alone have any idea what the proper fluids are.

If you care enough to have things like tranny fluid or diff fluid changed then care enough to try and do it yourself. If you cant then find a respectable certified mechanic to do it. The cost wont be much if any of a difference and at least that piece of paper says they knew at some point what they were doing.

Some of those "other" fluids too have special additives that in the wrong transmission can cause problems. Pennzoil being one i have seen first hand as bad for RX8 transmissions. I've used redline MT90 in other cars and its on my hit list for the next few weeks for my coupe when i swap the tranny and diff fluids out.

I have a 2010 G37S - manual transmission. At 14,000 the synchros in sixth gear started to go. I brought it in and they replaced the transmission - work order said "Found mechanical failure. Synch RO failure".