Variations is specified diff oil weight

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I posted a thread asking about this on a couple Dodge truck forums I frequent and didnt get much response. I thought I would post a condensed version here with a different question to get some opinions from the oil pros.

Basically, its coming time for me to change out the gear oil in my rear axle of my 2wd Dodge 1500. This will be the third change, the first being 75w90 (Royal Purple) that was listed as the correct fluid in Oreillys computer. The second being 75w140 (Mobil 1) that was listed to spec in the owners manual of my Uncles 2004 Ram 2wd.

Out of curiousity I started checking into this and broke out my falling apart 2002 owners manual and found that Dodge listed 75w90 for my 2002 1/2 truck, and in the 2003-on service manual, and the 2004-on owners manual it lists 75w140 except for 2500/3500 trucks, which should use 75w90. I was wondering why the change. Dodge 1/2 ton trucks are the same from 02-05. I have heard that the lighter fluid can be used if not towing, but why are the heavier trucks supposed to use 75w90, arent they expected to be the heavy haulers?

I am wondering about this from a cost stand point as well. I dont want to run the lighter fluid if the weight was raised due to a problem with it, but I also dont want to run the heavier fluid at the heavier expense just to drain good fluid out at 30,000 miles. The first 30,000 miles I ran the RP 75w90 it drained out with the same reddish purple color that it went in with. I also have an open diff if it matters. Any ideas or opinions are welcome.

Here are a couple shots from my 2002 manual:
IMG-20120122-00052.jpg

IMG-20120122-00053.jpg


It doesnt even say to use synthetic (which I will use anyway), just GL-5 Hypoid gear oil.
This is from the 2004 manual:

04manual.jpg


Here are some shots from my
 
I was able to research it a little more this morning. It seems that the 2002 trucks were sent out of the factory with the 75w90 and Dodge sent a letter out a few months later saying to change it out to 75w140 for towing applications, then they started putting 75w140 in starting with the late 2002 models on.
 
My 2002 Dodge 2500 4X4 quad cab long bed truck, with the H.O. Cummins, NV5600 6 speed manual trans, and limited slip Dana 80 differential called for (& got) 75W90 (I actually used RP Max Gear synthetic 75W90 w/a Mag-Hytec deep aluminum cover)-and it had ZERO problems towing anything I put behind it, never any problems. Max Gear is great stuff-the only thing you'll change with 75W140 is a fractional un-noticeable loss of MPG. I have a feeling your 1500 rear axle has a clutch-type limited slip diff as opposed to the 2500/3500 gear type Trac-Loc style-that's also why the manual recommends limited-slip additive (although RP gear oil doesn't need it).
 
I have an open diff, and with Dodges track record with the older ones and the retaining clip issue, Im glad of it lol.

I am going back with the RP 75w140 as I still have 1/2 unused bottle of it from the recent diff fluid change in my wifes F150. The 75w90 RP is the same price so instead of mixing I will just go with the same weight. Next change is scheduled at 200,000 miles and I will then go with Mobil 1 75w90 as I dont tow much, either that or Wallys 75w140 as I dont need the additive with my open diff.

I am wondering what made Diamler Chrysler change their mind mid year though on the spec'd fluid, and why the bigger axle is uses the same lighter fluid.
 
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