Comments on various Dodge/Cummins web-pages were all over the place when the subject of differential oils comes up. Anything from 75w-90 to 80w-140 were suggested for the Dana 80 regardless of Dodge's recommendations. Then I found this study which looks like it was at least sponsored by if not conducted by Amsoil: http://www.syntheticwarehouse.com/brochures/g2457_gearlube_study.pdf

If you scroll down on the chart to page 19 you'll see who the winner was (spoiler alert...it's Amsoil).

I've always stuck to an 80w-90 except when we were full time RV'ing. Looking at that chart I'm wondering if a 75w-90 full synthetic wouldn't work just as well if not better. Anyone?

 

Don’t see why a synthetic 75w-90 wouldn’t work, as long as it have the limited slip addictive in it or you add it the truck should be fine. 
im running Lucas 85w-140 in my 1500 with a Dana 70/60 swap. Mostly cause I ordered the wrong fluid from summit, but it’s been fine for 4yrs now without issue. 
 

I've had 75-110 amsoil in rear for 8 years now and 75-90 in front, probably need to change it soon, it has about 60k on it now and last time I checked it it looks exactly same and no metal on the magnet. 

Yeah, I'm looking at the Mobil One 75w-90 LS (limited slip) for the next change. It came in second in this study but is half the price of Amsoil. 

I'm so glad I changed mine on the 02 hundred thousand mile truck. The rear had obvious oil break down as it did not feel right when rubbed between your fingers,  felt like it had less lubricity and a pool of very fine grindings in the very bottom when running your finger down there. I cleaned all that out and used Dino 90wt. On the long trips that truck runs, making it easier to note any improvement, I noticed an improvement in mileage when I got back. I did the front also before leaving, wasn't as bad.

Edited April 10, 20214 yr by JAG1

I'm coming up on 200K miles on the rig so the tranny and differential will get flushed. They usually get drained every 50K but since the mileage is going up I might start changing them out every 35K or so. The truck is set up with a fairly heavy Caravan Camper shell (all steel) and other gear that stay in the bed all the time. We're retired now and take off camping at the drop of a hat. Anyway, it's a modest load so I don't see a need for an 85w-140 differential oil. I think the 75w-90 will actually run cooler and using a synthetic it should hold up 'til the next change. That study I posted above has me leaning towards Mobil's 75w-90 syn. 

I've never been a huge follower of oil studies. Typically the results go to the big brand names, AMSOil, RedLine, etc. 

 

There is other oil studies done here that people gone a long ways without big brand oil. Take to @cajflynn 1.3 Million miles and Never used any synthetic oils ever not to mention changed oil every 20k miles. Then there was @dorkweedthat ran over 84k miles on Walmart SuperTech Universal Diesel Oil 15w-40 and tested every 7k miles. Not to say the tested oils are bad but don't discount others. Seeing members running past 1 MILLION MILES mark and not using high dollar big brand oils is proof enough for me... Like @cajflynnjust Chevron Delo 15w-40 and Fleetguard filters nothing special. Like myself I'm either Chevron Delo or NAPA (Valvoline) for oil 15w-40 and NAPA filters right now.change interval of 10k typically. 

 

We've got another member here right at 950k miles now...

 I agree with @Mopar1973Man, alot of the studies I have read are skewed to the manufacturer sponsoring the study. Also, alot of the store brand oils such as walmart for example are made by better known manufactures so the quality is there just not the name brand.

 I won't knock amsoil because I do use it in my harley, it did make a noticable difference there but that is an air cooled V-Twin which see some pretty warm temps in the summer. Completely different animal compared to our trucks.

 The biggest thing is consistent maintenance in the form of oil changes or at least testing the integrity of the oil and good filtration.

IBMmobile's wife wishes he didn't have so many miles and was a well oiled machine. 

 

 

 

Edited April 11, 20214 yr by JAG1

2 hours ago, JAG1 said:

IBMmobile's wife wishes he didn't have so many miles and was a well oiled machine. 

Ya. she wishes for a lot of things. 

  

 

I tow a lot so I change the diff oil every 30k miles; it's cheap insurance.  The FSM calls for 75w-140 synthetic when towing a lot.  At Walmart I garbed some Valvoline with limited slip additive.   I have an AC system evacuation pump and set it up with some hoses and a jar so I can suck the oil out of the fill hole.

1 hour ago, IBMobile said:

 

I tow a lot so I change the diff oil every 30k miles; it's cheap insurance.  The FSM calls for 75w-140 synthetic when towing a lot.  At Walmart I garbed some Valvoline with limited slip additive.   I have an AC system evacuation pump and set it up with some hoses and a jar so I can suck the oil out of the fill hole.

I have a reversable oil pump and suction tube left from my old boating days. Thanks IBMobile for the idea as it will save having to redo the gasket ea. time.

 

I also used the Dino oil with limited slip additive but, had to add another 4 oz. to stop the chatter in when turning.

Edited April 11, 20214 yr by JAG1

That study I posted was obviously sponsored by Amsoil. Still, the results were interesting in that the 75w-90's seem to out perform the 80w-90's regardless of brand. I'm going to give the 75w-90 synthetic a try this fill. No way to know what's going on inside the differential without a temp probe but it's hard to believe a synthetic would perform worse than a dino oil.

I use 75w-90 Amsoil in my 4Runner and truck. The 05 was changed at 50K mile intervals and always looked new. The 18 will get changed a little more often until the warranty is up, then likely the same. 
 

The 4Runner was changed at 30K, then at 100K, and just recently at 200K (all odo readings, not intervals) and the used Amsoil lube is always in great shape. The lube with 100K miles on it felt like new lube. 
 

The factory fill on my 18 was toast after 15K miles, despite the 20K mile service life. So if you ask me the brand of synthetic does make a huge difference and it’s often with a few extra bucks, especially when you can reduce the services too. 
 

I need to do all the fluids in my Jeep and will be using Amsoil 80w-90 only over the 75w-90 because it’s cheaper and the Jeep only gets a few thousand miles a year and doesn’t tow anything. I’ll never have to change it again unless it gets contaminated. 

Edited April 12, 20214 yr by AH64ID

I just ordered some 75-90 for the front and 75-110 for the rear from amsoil. $20 for preferred customer gets you a good discount and order over $100 is free shipping. I also put on a magtech cover in the rear that holds like 8qts. I probably have 50 or so thousand miles on it (make it like 80  ) , not sure how many years but I know it's over 6 and it still looks new. ((( I just went out and looked when I got the mag cover.... It was in 2010  man time flies, this means I had it on for like 10 years with same oil )))  I've checked the temps with rf gun before and after I put mag cover on and I clearly remember improvement just don't remember actual temperatures. Last few years I haven't really towed anything but my boat. I debated on putting stock cover on to save on fluid but $60 whatever bucks every 6 years (10 lol) is not a huge deal considering oil now still looks good. I check the dipstick once a year and never had more then a dusting on it. 

Used amsoil in the engine too for few years but then switched to cen-pe-co that's made few hours from me and most Farmers use it in their equipment and sled pulling with good results, it's a parafine based and supposed to be great at high loads. Did few oil analysis and it's as good as amsoil. Where does time go? these gremlins I tell you...

()= Inserted after I realized how long it's been.

While I'm not a huge fan of Amsoil I broke down and bought their 75w-110 diff oil. I was going to go with Mobil One 75w-90 but thought the Amsoil would provide a little extra cushion with a loaded truck.

We just returned from a ~2000 mile swing through the southwest last week and I kept good track of mileage and fuel use. My rig with camper normally gets around 18mpg on one of these trips. The camper sticks up above the cab and provides plenty of drag. With the Amsoil though I got 20.25mpg. Changing out the differential oil was the only major change made so I have to give the credit to the 75w-110 oil. That's around a 10% improvement in mileage. The real surprise is how much headwind we encountered. I really expected my mileage to drop somewhat but it went up. 

I'll have to keep track of this on our next trip but I'm liking Amsoil's 75w-110 oil right now. Your mileage may vary! 

13 minutes ago, MikeH said:

While I'm not a huge fan of Amsoil

@AH64ID     

 

18 minutes ago, MikeH said:

I kept good track of mileage and fuel use

I have zero luck with consistent MPG.  I sometimes drive the same route, +550 miles round trip and the MPG is up and down 3-4 MPG.  Always running "empty", and I go the same speed.  I take into account the different fueling stations, when applicable - normally the same station, same pump when I can.  MPG calculated by hand.  Same trip, same fuel stations, same tire pressure, two weeks apart and the MPG is off by 3 MPG.  Probably even the same couple of XM radio stations.  I even note in my fuel log if I fight a headwind both directions, and mentally remember if it was hot one trip and colder the next.  I drive what some would call conservatively or like an old man.  Sometimes +/- 17 MPG, sometimes 18.5-19 MPG, but a couple weeks ago was 20.9 MPG.

I'm jealous you can tell the difference in MPGs on things like diff oil across a couple thousand miles, I have no idea what my truck's issue is with consistency.

14 minutes ago, LorenS said:

I drive what some would call conservatively or like an old man.

So you like running 75 or 80 too. Good to hear. I make a lot of road trips and try to end them as quickly as possible. Within reason anyway. At least thats how this old man drives.

40 minutes ago, dripley said:

So you like running 75 or 80 too

Maybe more like an old insurance agent?  But with a younger man's restroom break schedule, depending on coffee consumption.

 

Now that I think about it, maybe I drive like a guy driving to a slaughterhouse - both when I go to the meat packing plants, and when I'm driving home to two kids under 6, and the wife who is (rightfully) eager to turn over the reigns when I get home!

Edited June 15, 20213 yr by LorenS

One test doesn't mean a whole lot but that was the best mileage I've recorded since installing the camper. I use to consistently get into the low 20's when I had a tonneau cover on the bed. The camper killed that real quick but I wasn't going to gripe at 17/18 mpg going down the hwy. Just found it interesting that with wind advisories the whole trip and us seeming to be headed into the wind most of the time that the mileage would improve. 

We have a few more road trips planned this summer and fall. I'll let you know if this was a fluke or I'm on to something. For right now I'm leaning towards the lighter weight Amsoil as being the main factor. Nothing else has changed and we had a pretty good load in the bed (water, food, tools, etc). The empty weight on my rig right now is 8300lbs with that custom camper from Caravan.  

5 minutes ago, MikeH said:

I use to consistently get into the low 20's

I often envy you manual transmission guys, until stop and go traffic for a couple weeks, with no distant drive to a job site!

Kind of like I threw out the Dodge FSM and went against everyone and using Mobil 50 SAE Transmission Fluid in my NV4500. Way better lube than factory fluid and cost much less even for 5 Gallons. It is a GL-4 transmission lube and has zero impact on shift quality. This is the very same transmission lube used in Eaton Fuller 10 speed transmissions. Very heavy duty transmission lube.