Using "limited slip" Castrol GL-5 -> same as using friction modifier additive?

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Nov 11, 2020
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Ontario, Canada
The manual for my 2001 Dodge Ram (2-wheel drive, limited slip rear differential) calls for an 80W90 API GL-5 and also a "friction modifier" (about 50 ml).

Castrol sells a gear oil, Axle Limited Slip 80W90 API GL-5, and says that it is for "cars, trucks, motorcycles, atv's, scooters". Regarding "limted slip", on the back label it says only this: "Meets the service fill requirements of conventional and limited slip differentials ".

My manual calls for a gear oil that "should have MIL-L-2105C and API GL 5 quality specifications" but nowhere on the label is there any mention of any specification other than API GL5.

I understand that it is somewhat important for limited slip differentials to have the appropriate friction modifier, and I see other gear oils on the shelf that do not mention or have the term "limited slip" on their label. But this Castrol does. Should I assume it will meet the specs I need?
 

CleanSump

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MIL-L-2105C is just the DoD specificaton equivalent of GL-5. It has nothing to do with limited slip. It is also a superseded (old) spec.
 
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From memory of a friend having the same question. You can always try it, if the rear end chatters driving in a circle, add the additive, if it doesn't, drive on.

I think there are many manuf's that just include it in the additive package. It doesn't hurt a non-LS diff for it to be in there, and it keeps people with LS diff's from having to purchase separate additive.
 
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Yes, it's premixed for limited slip, and it's the way to go. You don't want to have to deal with the smell of opening and adding that additive.
 
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If you have not changed the fluid yet, is it chattering? Find a tight radius turn (preferably to the right if IIRC) and test it. Learn what it sounds like when the additive fails and the clutches chatter, so you know the limits and can spot it in the future.

The supplemented fluid may or may not have enough additive. Leave a few ounces of room in the diff, go test it. If not, then add some extra from the concentrate bottles. If it's good, then top up with gear oil only. On my truck, the supplemented fluid doesn't provide enough so I have to add more concentrated additive. YMMV.

On my truck I've learned the best way is to put in two containers of LSD additive, and then top up with straight (non-LSD added) gear oil. That's generally good for 40k until I can hear chatter on tight right turns. That's MY solution and yours is going to be unique to you but you need to be flexible and test to see what works.
 
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SumGuy

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I have drained the differential but haven't refilled it yet. This is a 2001 2WD Ram, with 45k miles, and the original diff fluid. Instead of taking the cover pan off I used a hand transfer pump to remove the oil, about 2 liters. I'm going to refill with the Castrol. I'm not sure what the chattering would sound like if I needed the modifier. The tires seem to normally chirp or hop a little when going around a corner, I assume that's normal and it not the chatter being mentioned here.

If I think after this change that I'm hearing chatter and was going to add the modifier, it looks like my first choice would be this: "CRC Posi-Trac Limited Slip Gear Oil Additive" (200 ml tube, $18).
 
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M
I have drained the differential but haven't refilled it yet. This is a 2001 2WD Ram, with 45k miles, and the original diff fluid. Instead of taking the cover pan off I used a hand transfer pump to remove the oil, about 2 liters. I'm going to refill with the Castrol. I'm not sure what the chattering would sound like if I needed the modifier. The tires seem to normally chirp or hop a little when going around a corner, I assume that's normal and it not the chatter being mentioned here.

If I think after this change that I'm hearing chatter and was going to add the modifier, it looks like my first choice would be this: "CRC Posi-Trac Limited Slip Gear Oil Additive" (200 ml tube, $18).
I believe there were previous cautions of adding too much additive, your gear oil should have all you need.
 
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VA
Not a Dodge, but Ive used Mobil1 75w90 in my Suburban LSD for almost 300k with no additive.
 
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