AMC 20 diff question for the petroleum experts

I noticed that the shop used a 140w diff. gear oil, instead of the 80-90w oil recommended by the '69 factory manual, and also an additive for the twin-grip.

I have read that one doesn't need the twin-grip additive if the diff. oil meets GL-5 standards.

So, two questions:

1. Is it better to run 80-90w gear oil, rather than 140w?

2. Is it better to run a GL-5 std. oil and eliminate the additive?

The facts -
* GL-6 is OBSOLETE and was NOT for limited slip unless it stated for limited slip on the bottle.
* "Twin grip" (a trade name) or limited slip (the generic name, covers almost all of them) need a special set of friction modifiers - often LESS than what's in many other gear lubes, or a modified friction modifier - which is NOT covered at all in the API GL rating system.
* GL-5 OR GL-6 with the right ADDITIVES could be specified for limited slip. It's the FMs, not the gear lube spec - GL - gear lube and the number is the criteria that the base lube meets, not the additives within.
* Get a good GL-5 that says "limited slip" right on the bottle.

(I run a synthetic 75w-something in my Javelin twin grip AND both Eagle differentials)

The rest of the story -
The "GL" rating is nothing to do with the limited slip part. That's the specs of the lube itself, extreme pressure characteristics, etc.
GL-5 with modified or reduced friction modifiers is fine if the basic spec of the differential itself calls for GL-5

There are anti-wear additives, friction modifiers, etc. The standard FMs in gear lube can wreak havoc with limited slip differentials due to the additives reducing friction of the plates - causing them to slip and not grip. Check the "fine print" - some GL-5 and GL-6 bottles say to NOT use them with "wet clutch" systems as in motorcycles, etc. - and that would include twin grip differentials.

A GL-5 may be labeled for limited slip or not labeled. If you don't see the limited slip part, then walk past that one until you find one with limited slip somewhere on the label. 
GL-5 is FINE in our "open" differentials. GL-5 with the modified additives if very fine for our "limited slip"
differentials.

YOU NEED THAT ADDITIVE unless the lube container specifies that it's for limited slip (and most these days do) -
here's a sample from a label - this one DOES meet the needs and notice carefully that they specify the differences -

"May be used where  either >>GL-5, SAE 80W-90 or GL-5 limited slip SAE 80W-90 gear lubricant<< is called for"
Now why would they specifically state each as if they are different?
Because they are different.
Both are GL-5 - but the ADDITIVES are different....
The "weight" or viscosity is also nothing to do with differentials or not. It's the EP or hypoid gear rating that makes it a gear lube for differentials. If it's not EP or hypoid, it's fine for your manual transmission but not your differential.

If it does not state it's for limited slip you need an additive but these days that's not too often. Most of us still think old-skool on these lubes and additives and with wet clutches being so common these days they often simply include the correct friction modifiers and additives and label the lube for limited slip.

GL-6 is inactive as are GL-1 through GL-3.
Only GL-4 or 5 are "active" today.  The GL-6 was NOT for limited slip but was for differentials with a more radical pinion to ring gear offset - where the pinion sat even LOWER below the centerline of the ring gear than is typical. It had extreme galling protection, etc.
Basically the only differences between 5 and 6 are that API Category GL-6 designates the type of service characteristic of gears designed with a very high pinion offset.
This design typically requires more gear scoring protection than what is provided by GL-5 gear oils. (The original API GL-6 test equipment is obsolete.)
GL-5 and GL-6 have ZERO to do with limited slip - it's the specs for the lube itself.

Look for GL-5 that states it's for limited slip - "gear lube" used in differentials must be labeled EP or Hypoid for use in differentials - as opposed to that which could be used in transmissions.


Edited by billd - Sep/09/2014 at 7:33am

In the shop we use Royal Purple Max Gear. It meets API GL-4 and GL-5, friction modifiers are included, and it is formulated for limited-slip differential use . No buying additives, no remembering to mix in additives, just fill and go.

http://www.royalpurpleconsumer.com/products/max-gear

80W90 for normal use

45W140 for towing, heavy loads, or extreme temperatures.

Most any oil on the shelf will be Gl-5. That's what you want anyway.

Some will say they already contain friction modifiers for limited slip, but in my experience they're still not slippery enough and need a bottle of modifier anyway. Depends on how grippy you want things to be.

The Wagoneer with a 2-spider trac-lok worked well on just the oil when it was worn out, but it needed some extra additive after I rebuilt it because it was so tight.

The Marlin has a 4-pinion powr-lok that has never been apart. I put oil and additive in it and it has always gripped when needed and slipped when not. I love the 4-pinion.

FSJunkie wrote: 80W90 for normal use 45W140 for towing, heavy loads, or extreme temperatures. Most any oil on the shelf will be Gl-5. That's what you want anyway.

Yup - he's got it. Good response on the viscosity.

As far as limited slip or twin grip, etc. - some will depend on the year and thus style or type, but in my experience with 1970, no modifiers needed beyond the lube I used. It's a synthetic labeled for use with "limited slip".

File under "just a thought" -
What you MAY want to do is try it with JUST the lube with no additive - then add additive if you have any odd sounds or it doesn't grip, etc.
That way you've not wasted money or additive, just the extra 10 minutes to put it in later if needed.  They've got a ton of money invested in the FMs that go into the GLs (friction modifiers and gear lubes) and I've PERSONALLY not needed to add any since the 1980s.

Matt's experiences were different than mine over the years and with different vehicles (he is also pretty logical in his response here, too) - but my car is a 1970 with a twin grip 20, and my past cars have been earlier versions of twin grip. Most of those I worked on up through the 1980s when I still ran my own shop were prior to the modern lubes so the lubes weren't made or labeled for twin grip/limited slip.

I suspect (and hope) in any case you have enough info to make an informed choice. Matt's info on the viscosity or "weight" was solid.
(and I've got a couple differentials in various stages of being rebuilt if he's in the neighborhood and wants to lend a hand)