What kind of gear oil for 8 3/4? Why not synthetic gear oil?

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daves66valiant

daves66valiant

68 Dart 340/727:66 Signet Vert 340/5spd: 68 D100
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I have a recently rebuilt 8 3/4 rear with new Richmond 3.91 gears and a Detroit Truetrac (no clutches or cones...all gear driven) limited slip differential done by http://coastdriveline.com/ These guys are awesome BTW and they suggested not to use any systhetic gear oil at ALL. I totally respect their advice but I had already put Redline 75W90 synthetic gear oil in the differential before I was told months later that synthetic is bad for my Mopar 8 3/4. Luckily the car has not been driven yet. ](*,)

Redline says its 75W90 Gear Oil contains a friction modifier which is recommended for limited-slip units.

Why is it recommended you not use synthetic gear oil in these old mopars with limited slip differential?

Just wondering about this before I drain the Redline stuff and replace it with non-synthetic gear oil.

Dave
 
Richmond 3.91 gears and a Detroit Truetrac (no clutches or cones...all gear driven)
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Maybe one of these manufacures suggested not to use it.
I have an OE sure grip in a 741 with Richmond 4.10 and have used dino for break in and a few thousand miles.
I needed to take the rear apart and have done nothing but full sythn. ever since. It's been many years since without problems.

Ask them directly.
 
I think I remember Mark the guy at Coast saying that the manufacturer recomends against it.

Thanks Rum

Anybody else?
 
I use nothing but synthetics and have been for over a decade. Anyone that tells you to not use synthetic is living in the past.
 
richmond says not to use synthetics because they don't mix with friction modifiers well. i had a long discussion with a guy at richmond about a 55 belair i was doing that had 1 of their trannys. he had problems with synthetics not mixing with the additive and noises coming out of the rear until you drove it a bit. i use lucas and haven't had a problem yet.
 
I installed a Auburn Gear limited slip differential in my Dodge Ram a few years ago and Auburn says to use non-synthetic gear oil only. This a cone type differential so the synthetic must be to slippery is all I can imagine. I believe the clutch type and cone type is of debate but the type differential that you have I don't think would make a difference.
 
First of all, synthetics and friction modifiers do work well together, Chrysler does it. I did it in my dually Dana 80 tow vehicle with out complications.

Secondly the type of differential he has doesn't need a friction modifier anyway.
 
Guitar Jones said:
First of all, synthetics and friction modifiers do work well together, Chrysler does it. I did it in my dually Dana 80 tow vehicle with out complications.

Secondly the type of differential he has doesn't need a friction modifier anyway.
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why wouldn't he need one? i was taught that all limited slips need a friction modifier. my previous info was straight from a tech at richmond gears.
 
The friction modifiers were developed to keep the cone and clutch type diffs from chartering, sticking and making noise. TrueTrac and other types of all gear differentials don't have that problem.


The Chrysler corporate rears with limited slips (9.25 and 8.25) still use a friction modifier with either 75w-140 (for the 9.25) or 75w-90 (for the 8.25) synthetic lube. The pre 2003 2500 and 3500 trucks with limited slip Dana rear ends also use the modifier and the synthetic for refill after service.

The current AAM axles (9.25, 10.5 and 11.5) use the synthetic lube but don't require the friction modifier for their limited slip differentials.

I hope this helps. The friction modifier isn't even a necessary additive for a cone type diff if you don't mind some noise and maybe a little jerking from the rear end at low speed parking lot type maneuvers.
 
I run Mobil 1 synthetic gear lube in all my rear ends (and front ends on 4x4). Mobil 1 does not require addtional friction modifier and I do not add it in the limited slip diffs and have not had any chatter problems.

When I rebuilt the rear end in my Barracuda I do recall that the Yukon gear instructions recomended a 500 mile break in with conventional gear oil before swapping to synthetic gear oil which I did. The rear has been trouble free and quiet for over 30k miles so far.
 
Guitar Jones said:
I use nothing but synthetics and have been for over a decade. Anyone that tells you to not use synthetic is living in the past.
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exactly what i was thinking.. old school thinking guy. if your that worried call detroit locker and ask them if its ok to run.
 
well that sounds good for all other rears, but the 8.75 needs what? my rebuilders use eaton/auburn units. i usually use the lucas. is that ok? i have always told customers to change it after 500 miles, but didn't know if synthetic was good or not.
 
I did some searching online and this is what I found from tractech the maker's of the detroit truetrac (an eaton brand)

Unlike the Truetrac, most limited-slip differentials like the one shown above operate like open differentials but have spring-loaded clutch packs to mitigate the otherwise unrestricted slip between the two sides of the differential. This is a far inferior design to that of the Truetrac. These clutch packs wear out and the differential becomes more and more like an open differential with use. Also, these differentials create handling problems on high-traction surfaces like the road because they want to remain tight and prevent differentiation between the two wheels. Finally, these clutch-driven limited slips require special friction modifiers to be mixed into the gear oil to permit the clutch packs to operate smoothly.

Tractech Truetrac Differential

The Truetrac is gear-driven, just like the standard differential, and operates on a completely different principle. The Truetrac operates like an open differential in high-traction conditions. On low-traction surfaces, the helical side gears prevent one side from spinning out of control while the other side remains stationary, as often occurs with an open differential. During times of severe differentiation when one side of the axle wants to spin out of control, the helical worm gears are pushed away from the larger helical gears and pushed into the carrier. The additional resistance on the side with less traction diverts torque to the other side of the axle. The gear-driven operation provides several dramatic advantages over the clutch-operated designs. There are no clutch packs to wear out on the Truetrac, and a Truetrac wears no faster than any standard differential. The operation of the Truetrac is also smoother and more transparent than the clutch-operated designs. Finally, there is no special friction modifier required for proper use, and standard 90wt gear oil is the fluid of choice with the Truetrac.

I also found a bunch of offroad sites that love the truetrac and they all suggest synthetic gear oil.

My driveline guy suggested 85W140 GL5 non-synthetic. Everything else I've looked at seems to differ. I think I'll just run the synthetic for now and check it after 500 miles. I still have time to decide but it makes since that I could use synthetic since their is no need for friction modifiers. The Redline stuff does come with friction modifiers already in it though. The debate goes on I guess.
 
I use Mopar synthetic 75W-90 in my 8.75 race car. Mostly because I get it for free. If I had to buy it I'd probably use Amsoil of similar viscosity.

The race car is equipped with a pro gear and a spool so no limited slip to worry about.

The tow vehicles have always had 75W-140 Mopar synthetic in them.
 
Mopar guy here uses REDLINE shockproof and has also used royal purple, but says Redline is the stuff!
 
Shainesboostin said:
Mopar guy here uses REDLINE shockproof and has also used royal purple, but says Redline is the stuff!
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Chrysler recommended that if u do heavy towing to change to synthetic in the 02 ram w/ a dana . That tru trac flat wouldn`t hold in a ford 9" in my 406 vega w/ 14 1/2 x 32 slicks on it , ended up w/ a full Detroit locker. jfyi
 
Living in the past strikes me as ironic.
Aren't we all living a little in the past here?
I mean, this is an antique car forum.
I always run Mobil 1 in my engines.
And I know from fixing planes that synthetics have advantages in some applications.
But if there has to be a big discussion about whether or not to even use synthetic in a hypoid gear box, doesn't that raise some red flags?
Are rear ends failing that often? I think not. It's worked well for how many years with conventional oil?
Is a gear box really subjected to the same conditions as a internal combustion engine?
Are the break down,contamination and corrosion protection requirements even remotely the same?
Seems to me this is one of those "it ain't broken, don't fix it" subjects.
That's all I got.
Except this.


 
I think the line of thinking comes from back when the "new" synthetic oils were coming out. Could clutches/cones be designed to perform properly with synthetics, yea probably, but with anything non-standard, the manufacturer would rather reccomend what they know works, vs doing the research and testing everytime a new oil type hits the market.

Just like how you have to turn off your electronics during take off and landing. Are you going to crash the plane with your Iphone? No probably not, but with technology advancing so fast, you never know, so the standard is to turn them off..
 
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