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Differential Oil

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go_inbroke

go_inbroke

Senior Member
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I am going to replace the oil in the differential of my 61 TR3A.

I have read about 100 opinions of a product to use. SPI GL4, GL5, MT1 etc......

Which is it that will protect the "yellow" metals in the diff?

Lucas' Heavy Duty claims it meets GL5 and MT1 standards.
 
Alas, all you'll get here are more opinions, to go along with the 100 you've already read.

None of the standards directly address the issue, in fact GL5 actually has the toughest test for "yellow metal" corrosion, and yet it's considered the "bad" spec. But MT1 does address long term, high temperature stability; so it basically rules out the sacrificial "active sulfur" additive that can cause yellow metal corrosion.

So, my opinion, anything that meets MT1 and GL5 will probably be good, both in protection for the gear faces and in not corroding the brass thrust washers.

I ran Valvoline's full synthetic gear oil (GL5, MT1) in my TR3A for nearly 20 years. The diff thrust washers were already well worn when it came to me, but did not get noticeably worse in perhaps 200,000 miles of mixed freeway and city driving plus the occasional speed contest. It works for me. (And I'm still using it today in my TR3.)
 
Just bought some EP90 oil for mine and I was advised to go for GL4 and not GL5, which apparently has an acid content to help it etch into modern gears, but eats yellow metal!
 
I am using Castrol Hypoy GL4. I bought a few quart bottles from TRF when it was on sale rather than chase down a 5 gallon bucket. I may get one later or switch to another brand later.

Advance has Coastal brand GL4, which I know some TR guys use.

Have fun changing it out!! I had never done that before. What a mess!
 
CinneaghTR said:
Have fun changing it out!! I had never done that before. What a mess!
Click to expand...
Did the solid axle 4As not have a drain plug for the diff? I'm pretty sure all the earlier cars TRs did, but thought it was only the IRS diff that didn't have a drain plug originally.

Of course it's not too hard to add one ...
DSCF0022_reduced.jpg
 
CinneaghTR said:
Yep, the 4A solid rear has a drain plug...
Click to expand...

ÂżVerdad? My TR4 diff has no plug -- and yes, changing the fluif is messy.

Maybe next time I'll add one (but not if the diff has to come out).
 
Geo Hahn said:
ÂżVerdad? My TR4 diff has no plug -- and yes, changing the fluif is messy.

Maybe next time I'll add one (but not if the diff has to come out).
Click to expand...
Shouldn't be too hard with the axle in the car; after you've removed the cover to drain the oil the old fashioned way. IIRC I used a 3/8 NPT tap and, lacking the proper 37/64 drill bit, a 9/16 bit with a 1/2" shank to fit my drill chuck. The cast iron housing taps easily (although I used cutting oil anyway), so a cheap carbon steel tap will work fine (about $6 @ Enco). Enco also has a 37/64 bit with a 1/2" shank for about $10.

And as usual, I recommend a magnetic, hex socket plug for the drain.

DSCF0025.jpg
 
CinneaghTR said:
This concerns gearboxes, but I found the disucssion of GL4/GL5 helpful when I was looking into this.

Buckeye Triumphs on Gearbox Lubricants
Click to expand...

Just a small side point : Redline MT-90 is specifically NOT recommended for use in differentials (in spite of its GL4 rating).
 
TR3driver said:
Just a small side point : Redline MT-90 is specifically NOT recommended for use in differentials (in spite of its GL4 rating).
Click to expand...

Do you mean not recommended by Redline, or by someone else. I was in a local outfit that does restoration work on British, and other, cars that recommended the MT-90.
 
Not recommended by Redline:

"<span style="color: #660000">MTL, MT-85 & MT-90 are not for use in differentials with hypoid gears</span>"
https://www.redlineoil.com/Products.aspx?pcid=7

And in case there is any doubt, Triumph differentials have hypoid gears.

Besides, why would you want to use a very expensive oil that is specifically designed for high friction, in an application where you particularly want low friction? Doesn't make sense to me.

Except of course for people who only focus on the "GL4 is good, GL5 is bad" myth without understanding it.
 
TR3driver said:
CinneaghTR said:
This concerns gearboxes, but I found the disucssion of GL4/GL5 helpful when I was looking into this.

Buckeye Triumphs on Gearbox Lubricants
Click to expand...

Just a small side point : Redline MT-90 is specifically NOT recommended for use in differentials (in spite of its GL4 rating).
Click to expand...

Got it, thanks! I was not thinking of Redline, but that is good to know.
 
TR3driver said:
Besides, why would you want to use a very expensive oil that is specifically designed for high friction, in an application where you particularly want low friction? Doesn't make sense to me.
Click to expand...

Randall,

At this point it is getting to where we need to be looking at what options are available, as GL4 is getting harder and harder to find. I have plain old GL4 in my TR4 differential now, and a little more on the shelf. From the stories that others are telling, it seems like GL4 might not be available for very much longer.

Also, can you expound a bit about the high friction vs. low friction applications? I am not sure I am following what you are saying. Do you mean that MT-90 is a high friction oil? I don't know if that makes any sense to me, but I am open to an education...
 
Redline's MT and MT-90 gear oils are formulated specifically for synchronized manual transmissions, which rely on friction between the synchro rings and the gears for proper operation. Somewhere on Redline's site, there is even a curve describing how the coefficient of friction changes with speed, which helps the synchro rings "grab". The difference in synchro operation is quite apparent when you switch.

But for a differential, a higher coefficient of friction just increases power loss and heat. Unless it's a limited slip, you want as 'slick' an oil as possible.

Hypoid gears (the ring & pinion in a normal differential) are particularly difficult to keep lubricated, because of the very high surface loading at the gear face (which literally produces a local deformation of the steel), and the wiping action caused by the gears not turning on the same axis. That's why they need "extreme pressure" (aka EP, aka "hypoid") oil.

And the Valvoline full synthetic GL5 works just fine, for half the cost of the MT-90.

I believe it actually works better than conventional GL4, by reducing heat (and power loss). There was a test report in Road & Track a few years (decades?) back, where they did head-to-head tests with conventional and synthetic oils. They actually measured about 7% more power at the rear wheels, just by switching to synthetic oil in the engine, transmission and differential. The power gain went away when they switched back. 7% may not seem like much, but that would be about 7hp behind a 100hp Triumph engine, and just a 2hp loss is about the equivalent of a portable electric heater. Imagine what one of those would do inside your differential!

Of course, if you only drive your TR in parades or on and off the trailer, then likely none of this matters. But some of us do drive hard on occasion, and even just accelerating away from a stop sign puts a lot of torque on the diff.
 
Randall,

Thanks for the added info. It was very helpful. I use the Redline MTL in the transmission of my BMW and it works quite nicely. I use the Redline 75W-90 (or 80W-90, I forget which it is)in the diffential of my BMW.

Besides the "high" cost, do you think the Redline gear oils would be good for a TR differential?
 
Oh, and to corroborate your point about the need for friction in the syncros of a manual transmission - I tried Redline ATF in my BMW transmission (it is recommended as one of the acceptable lubricants for this purpose) and I had trouble getting the transmission into lower gears without good rev matching. I recently changed the oil to MTL and all is back to normal.
 
I use Sta-Lube GL4 80w/90. You'll find quarts at most NAPA stores. Also had a drain hole drilled and tapped when Quantum Mechanics rebuilt the diff.

Also, use Redline MT-90 in OD gearbox. Was using SAE 30 non detergent but it was very notchy in 3rd gear.

Bob
 
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