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Manual Transmission Gear Oil - 85 Nissan 720


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Going to be doing a gear oil change in the next few days, trying to find an appropriate GL4 oil, it looks like I might be able to find Redline MT90 at some select specialty shops here in San Diego, CA, so I am looking at some other options for gear oil. So far the one option I have found this evening was that Autozone stocks various weights of Valvoline GL-4, now I have no idea how good this stuff is and was wondering if anyone has used the stuff, they also have the Lucas brand, but I don't know much about there stuff. The other question is is the Redline MT90 worth the effort in finding a specialty store that stocks the stuff?


Side note: Any recommendations on a rear diff gear oil?

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Worth a read as GL-4 is getting much harder to find as our old transmissions are, well OLD and newer ones don't use it. Seems everyone raves about this AC Delco oil. You'll note in the text it's marketed by Shell and even Chrysler uses it under it's own label. I'm thinking of trying it this year. At $10 a liter (you need two for your 71B) it's no more than what I pay now.



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When I swapped the old GL-4 90W out years ago the transmissionwas 'stiff' to upshift into second when starting off cold from the driveway. It goes away as the trans warms up. Warm downshifts to second and specially first, were slightly stiff and sometimes a slight grind as they nestled together. Not a big thing unless you were in a hurry. I replaced the oil with a semi synthetic 90W that appeared thinner than what had come out. The cold shift to second is effortless now and the gears make a quicker, positive, metallic CLICK sound when shifted that I find reassuring and mechanical.


This was in my first year '79 zx 5 speed that I scored with unknown mileage on it and put in my 620 when it failed. It's been across Canada and back 8 times at least and probably has 60K miles on it just from me. It's now in my 710 goon, so on it's third vehicle. 


I've heard that 40W engine oil is about the same viscosity as 90W GL-4 and will work. It won't have the proper anti scuff aditives but....


ATF is about 7-12 W (there is no need for a W rating so is a guess) I discovered this when looking for a hydraulic oil for my struts. I had ATF in my new '70 Dart so it could be shifted in cold Ontario winters. The bearings failed and they were replaced under warranty and guess what? The dealer refilled it with ATF so I replaced it with 90W. On cold nights I had to leave in neutral or you couldn't take it out of gear to warm it up in the morning. Not kidding, you could not pull it out of gear! ATF is (I think) a little thin.

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  • 4 weeks later...

On a particularly cold night, around -40°F

I went to start my truck. Let my foot off the clutch (in neutral) and the truck rolled forward up a slight incline and stalled.


The gear oil was so cold and thick it engaged the gears. Not sure what weight it was, but I changed it out for some 75w90 Royal purple synthetic and it never did it again.

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The gears didn't engage. Just think of an electric fan blowing on another fan. The air from one will spin the blades on the other. Now instead of air... think molasses.

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I have the GM AC/Delco syncromesh gear oil in my trans and diff. Got it for free out of the pumps when I worked at the Chevy stealership. It is some good stuff. Been running it for 3 years so far. Shifts a lot smoother than the blackened goop that was in it, that's for sure! Supposed to be long life up to 100,000 miles as well, although I dunno if I want to wait that long.  GM does do some good reasearch on vehicle fluids. You would be surprised how much we had to learn about the full synthetic "DEXOS" motor oil when it came out. Most of that was to explain to customers why the price of an oil change doubled, however. Then again so did Ford when I worked there. Like for example when they switched most engines to 5w20 for fuel economy, and why all their manual transmissions run ATF.  Than again, Honda used to recommend using 10w30 motor oil, so go figure?

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Differentials are a totally different gear that only uses GL-5. Transmission oil won't have the proper anti scuff additives in it. Does the GM oil say it can be used in differentials????

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You're right Mike,  never mind! I used the GM Manual Synchromesh in the transmission. It is a specific trans fluid. I used GM's "synthetic axle lubricant" for the differential. I realized that after I looked it up the Sycromesh on GM's parts website. It has been 2 years since I worked there, more than 3 since the fluid change. I remembered the diff fluid after I read the description where it says "grape scent." It really does smell like grape, kinda nice from the usual stinking worse than anything imaginable gear oil. We mechanics loved the stuff after they switched the shop pumps over to it!



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Redline MT90 is specifically designed to work with brass synchros. I use it whenever I can. The difference in cold shifting is vastly improved with the MT90.


Read through their spec sheet and decide for yourself. http://www.redlineoil.com/product.aspx?pid=46

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  • 1 year later...

Maybe I'm a little ignorant but I have a gallon of valvoline 80w90 gear oil. It's not synthetic, I use it on my four wheelers differentials. On my 85 720 I already drained my rear end and filled it with this 80w90 I have. On the back of the jug it says it meets the specs of GL4 AND GL5. So I'm I crazy in thinking it will work fine in my transmission? I'm going to drain it tomorrow and re fill it.

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There have been mixed information regarding if it's truly safe for brass. I went down that rabbit hole and there were some responses from the manufacture that claimed it was safe. But really, for $20 you can get GL4 from napa. Sta-lube gl4.

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