Yes, a Dumb Question: Rear Diff Fluid

[cervelo15]

Hello all friends! I did a search for this, but can’t seem to get a definitive answer. I’m changing the rear diff fluid in my 21 Ram with 3.92 limited slip diff next weekend. I purchased 2 bottles of the additive, but I’m really confused about the rear diff oil to use in it because of the limited slip additive content already in some of the oils.

I was hoping to get Amsoil, but its looking like I won’t be able to get it in time. I saw Valvoline has Synpower, but when I read it, it says it already has limited slip additive in it. I just don’t know how much.

Will I have issues if I put the a diff oil with additive in, plus the 5oz of additive? Is the content in the diff oil negligible enough that adding the Mopar additive on top of it will be ok? We re heading on a road trip to NC in a few weeks, so I want to avoid any issues when we re miles away from home.

Thank you all for your help and ideas!

Edit: just found this, but not sure how much I trust them telling you not to put additives in:

 

[Burnzie]

Why are you changing it, not sure what is recommended in the owners manual as to miles on it,but you have a 2021my can't have that many miles.

 

[HemiDude]

Since that particular Valvoline is made for Limited Slip diffs, I would think that you don't need to add any.

 

[Scram1500]

The additive is designed for use with Mopar differential oil

 

[HSKR R/T]

From my experience on older Mopar trucks. The additive already in most diff fluids isn't sufficient for the Mopar limited slips, and you still need to add more.

 

[GKIII]

Don't put any additives in...then if it chatters add a bottle of friction modifier.

 

[cervelo15]

Why are you changing it, not sure what is recommended in the owners manual as to miles on it,but you have a 2021my can't have that many miles.

well, for multiple reasons: approaching 20k miles, saw stories of low fill from factory, and peace of mind knowing whats in there because I put it there.

 

[theblet]

well, for multiple reasons: approaching 20k miles, saw stories of low fill from factory, and peace of mind knowing whats in there because I put it there.

Have the shop check the level when you have the oil changed. If it’s low you’d probably hear some whining from it too

 

[cervelo15]

So I did my homework and checked local dealers for Amsoil…after a few phone calls, found a goldmine of Amsoil products at a local repair shop. And they were cheaper than website…got some grease too! So now I’m all set for this weekend to change the rear diff oil.

 

[GKIII]

So I did my homework and checked local dealers for Amsoil…after a few phone calls, found a goldmine of Amsoil products at a local repair shop. And they were cheaper than website…got some grease too! So now I’m all set for this weekend to change the rear diff oil.

Just an FYI, you can use a gasket rather than rtv. Much easier, no set up time, and don't have to remove sealant when you change it the next time. I use these with no issues:

https://www.amazon.com/dp/B01LYEAPL0?ref_=cm_sw_r_cp_ud_dp_CS7GWC7N5DPA3Y41VNSC

 

[HemiDude]

Just an FYI, you can use a gasket rather than rtv. Much easier, no set up time, and don't have to remove sealant when you change it the next time. I use these with no issues:

https://www.amazon.com/dp/B01LYEAPL0?ref_=cm_sw_r_cp_ud_dp_CS7GWC7N5DPA3Y41VNSC

I guess I have never paid attention, do these ram rear diffs not have a drain plug?

 

[jmt8706]

20K miles is still pretty low, but do whatever you want, it's your money.

 

[cervelo15]

Just an FYI, you can use a gasket rather than rtv. Much easier, no set up time, and don't have to remove sealant when you change it the next time. I use these with no issues:

https://www.amazon.com/dp/B01LYEAPL0?ref_=cm_sw_r_cp_ud_dp_CS7GWC7N5DPA3Y41VNSC

Thanks, this is good to know and I did see a video from someone who used this. I’m not pulling the cover off this time, but when I do, I ll keep this as a necessity to put it back together

 

[cervelo15]

20K miles is still pretty low, but do whatever you want, it's your money.

I figured its way early (especially since I baby the truck and don’t drive a
lot, but this is a two fold exercise. Checking to make sure the level is sufficient and getting another maintenance item experience under my belt.

plus I can fill it with a high quality gear oil

 

[GKIII]

I guess I have never paid attention, do these ram rear diffs not have a drain plug?

They do, but if you wanna give it a good flush (and remove the pulverized metal on the magnet) you have to remove the cover. Plus I like to look for any signs of premature wear while I'm under there.

 

[jmt8706]

I figured its way early (especially since I baby the truck and don’t drive a
lot, but this is a two fold exercise. Checking to make sure the level is sufficient and getting another maintenance item experience under my belt.

plus I can fill it with a high quality gear oil

I guess if you have time to do it now, not a bad idea to give it a shot like you said.

 

[cervelo15]

I’m happy to report I completed the fluid flush and fill for the rear diff….was scared out of my mind doing it, have no idea why (maybe because this was the first ever time I did it). Had to run to auto store in the middle of it to get fluid transfer pump because my squeeze bottle idea didn’t work (and they didn’t have the squeeze bags of fluid available). Those things work awesome, pushed in a quart of oil in no Time.

The bad: mine was way low From factory. Fluid didn’t even flow out of the fill plug when I removed it. Stuck my finger in and nothing. Disposing of old fluid, I didn’t even get a total of 2 qts and a smidge and a half more (maybe 2.1 quarts). No metal on the plugs. Fluid looked like mud.

The good: refilled it until it flowed out of the fill plug with high quality Amsoil with the Mopar friction modifier. I did my best to get the 5 oz of the friction modifier, so it might be a small +or- margin of error.

Took it for a 10mi round trip ride to get gas and it seemed like it didn’t have the hesistation it used to. Pretty smooth too. I didn’t hear any noise, but I have no idea what I’m listening for anyways if anything was bad. Also checked at the gas station and didn’t see any leaks From underneath.

Only question I had was about the thread sealant: I used the Permatex high temp thread sealant, but the stuff that squeezed out didn’t seem to Dry. Does the stuff ever dry and harden to make a seal? Also, is there a torque spec for those plugs? Or is it the same as an oil plug, don’t tighten the crap out of it?

 

[HSKR R/T]

I’m happy to report I completed the fluid flush and fill for the rear diff….was scared out of my mind doing it, have no idea why (maybe because this was the first ever time I did it). Had to run to auto store in the middle of it to get fluid transfer pump because my squeeze bottle idea didn’t work (and they didn’t have the squeeze bags of fluid available). Those things work awesome, pushed in a quart of oil in no Time.

The bad: mine was way low From factory. Fluid didn’t even flow out of the fill plug when I removed it. Stuck my finger in and nothing. Disposing of old fluid, I didn’t even get a total of 2 qts and a smidge and a half more (maybe 2.1 quarts). No metal on the plugs. Fluid looked like mud.

The good: refilled it until it flowed out of the fill plug with high quality Amsoil with the Mopar friction modifier. I did my best to get the 5 oz of the friction modifier, so it might be a small +or- margin of error.

Took it for a 10mi round trip ride to get gas and it seemed like it didn’t have the hesistation it used to. Pretty smooth too. I didn’t hear any noise, but I have no idea what I’m listening for anyways if anything was bad. Also checked at the gas station and didn’t see any leaks From underneath.

Only question I had was about the thread sealant: I used the Permatex high temp thread sealant, but the stuff that squeezed out didn’t seem to Dry. Does the stuff ever dry and harden to make a seal? Also, is there a torque spec for those plugs? Or is it the same as an oil plug, don’t tighten the crap out of it?

In future, always add the friction modifier first. Then top off with fluid.

 

[cervelo15]

In future, always add the friction modifier first. Then top off with fluid.

Will do! I thought mixing it together would help. I didn’t do any damage right?

 

[HSKR R/T]

Will do! I thought mixing it together would help. I didn’t do any damage right?

No, you will be fine. And if you add it first it will mkx as you dump in the rest of fluid, and won't take but 10-20' of driving to be thoroughly mixed together.