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Transmission Fluid and Differential Fluid Change


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I am going to change the trans fluid and differential oil in my 1985 720 2WD 5 speed manual. From what I have gathered, I should use the following:


GL-4 75W90 for the transmission - 2.5 qts, roughly.


GL-5 for the differential - 1.5 qts.



Are these quantities correct?  What weight do I need for the differential? Can I use GL-4 in the differential?  I would like to be able to just buy a gallon of gear oil and use it for both, if possible.


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80w90 GL4 is the same thickness as 80w90 GL5 and would shift the same. However differentials need extra scuff protection on the tooth faces that transmissions do not. This anti scuff additive contains a sulphur compound (you may have noticed the smell when changing the oil?) that in GL4 is in an amount (about 4%) that does not bother the copper/brass.bronze synchronizer rings. GL-5 oil contains it in about 6% concentration, which is 50% stronger and will over time tarnish and erode the copper.


GL4 is more than enough protection. GL5 is not good for Datsun transmissions.


Even GL4 should be changed at the recommended intervals. About 5 years 30K miles

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I picked up a gallon of GL-4 only Sta-lube from Napa (they only had one gallon bottle, no quarts) and a quart of GL-5 for the differential.  It was about 15 degrees all weekend so I didn't make it over to my unheated garage to change it out.  Hope to get it done this week when the temps are supposed to get back up into the 50's.  Thanks for the replies.

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Your transmission will thank you for the GL4 later! 


I am interested to see what the fluid that is in the truck now looks like.  I can't imagine it is good.  I got the truck for $250 and it was in rough shape.  I have only put about 3000 miles on it in 9 months, but it has come back from the dead nicely.  There is a slight whine in the trans (I think that's where its from) at 50 mph and above and I am hoping new fluid will help it out.  I use it as an in town parts hauler and runs very well considering where it was when I got it.

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Me too, usually.  I was so surprised I was able to get this running and driving that the trans fluid slipped my mind.  It has had oil changes, a coolant flush and a brake fluid change.  I haven't changed the blinker fluid, but it's on the list.

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30K miles is the usual change time so look at the mileage and imagine that the first owner may have done this. Second owners and successive ones don't want to sink money into a 'junker' and skip this.


Generally it will be thick (90w) and dark but clear honey color with a sulfurous smell. The drain bung has a magnet on the end that collects ferrous metal. A black paste is normal. Clean this off so you have something to gauge next oil change. 

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Go the trans fluid changed last night.  It was low, but there was no noticeable leakage from anywhere.  New fluid went in fine and it seems easier to shift, seems to engage more directly or something.  Glad I did it.  The old fluid was thin and pretty well black in color.  The magnet on the plug had some paste covering it along with a bit of metal shavings, but not bad.  The hardest part of the job was waiting while the honey-thick new fluid filled the transmission.  I used a length of tubing and a funnel and filled it from the engine bay.  Poor enough to fill the funnel, wait, repeat.


Hope to get the differential fluid changed tonight.


Next step is to chase down the oil leak that I have from somewhere about the oil pan (and maybe the oil pan as well).  I will need a bunch of cleaner as the entire passenger side of the engine is covered in oil.  leave a small stain overnight, but does not leak enough that I have needed to add oil.


After that I need to track down the problems I am having with the turn signals - they only work if I hold the lever down to the spot before it clicks to engage.  If it goes all the way to the click, they don't work.  And if the left signal is used, the brake light shorts out until the truck is turned off and restarted.

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  • 2 years later...

No up date on your signals? I just bought a 1980 720 2wd. Replaced my turn signal switch the one on it snapped...


When I signaled to turn right nothing. When i signaled to turn left all 4 came on.

And no head lights.



Got head lights working ( shorted wire)

Now turns wont work at all. 


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  • 3 months later...

Bumping this thread with some new transmission fluid questions...


GL-4 is best for 4X4 with a 5-speed as well?


I’m getting some solid CLUNKING and GRINDING when going into first and reverse...


Syncros seem to be going and I’m sure a full transmission rebuild is in my future...but I’m hoping to buy some time with a fluid change.  


Any specific brands that I should be seeking out?  I’ve heard that Swepco fluid is particularly good for addressing fussy synchros...

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80w90 GL4 for all Datsun/Nissan transmissions. Synthetic, if compatible with  copper or 'yellow metal alloys' is also good but expect it to seep past old seals like on the tail shaft. A new seal is about $5 and an hour's work to remove the driveshaft and put in. I'm using GM's Delco Friction Modified Synchromesh Transmission Fluid, a synthetic. It's definitely a thinner viscosity high performance transmission oil for the Camaro/Corvette and safe to use on brass synchro rings. Shifts faster with the same effort. Shifts smoother when cold. Down shifts now where it would grind. This will not fix a transmission that is worn out and a waste of money.If you're sure it's got problems a thicker 80w90 will help you limp by.


I imagine this stuff is the same as those other synthetics but with a different color added. Expensive but well worth it to me. $18 Canadian so $14 US a liter and you need two.


Place in neutral let clutch out, then clutch in count to 3 and shift into reverse. If it grinds your clutch is rubbing or not fully releasing. About all you can do is check the clutch master is full.


There are 4 U joints between the transmission and the differential. Inspect them closely, they should turn only. They should not have any play or move radially to the driveshaft. A bad one will clunk.





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