I see the manual for my 2004 3500 SRW calls for 75W90 SYNTHETIC gear oil in the front and rear differentials. AAM type 11.5 inch rear.

My mechanic says it makes no difference if it is regular or synthetic as long as it is 75W90.

Why would Chrysler want synthetic only gear oil.

Is there any good reason why not to use just ordinary 75W90 gear oil like the mechanic says??

 

synthetic gear oil withstands high temps better than conventionals. your rear diff gets pretty hot during high speed high load towing. they dont want the oil breaking down and allowing component failure due to lack of lube.

 

X2- I also remember something floating around about using 75W140 for the rear.

 

if you are doing alot of really heavy towing, 75w140 will help.
or put a cover on that helps with cooling due to fins and increased capacity.

 

Dodge (or any other manufacturer) would do anything to save $1 on the manufacture of a vehicle. If they manufacture these with synthetic oil installed, there must be a reason.

 

Yup, like it was said earlier, synthetic gear lube does not break down like conventional does. I would HIGHLY recommend you stick with synthetic. If you are doing a lot of towing, I would run 75w140, but if you run empty a lot of the time than 75w90 works great. I use Amsoil severe gear 75w90, but I run empty about 98% of the time, and only tow 2-3 times a year.
Scott

 

The move by Dodge from 75/90 to 75/140 gear oil was only for the 1/2 ton pickups. I know when I first read about this a few months back I had my local Dodge dealer look up the information and show me on their official link to Dodge.

 

Usually "Mechanics" are not engineers.

 

I was told that the 140 would affect your mpg's negatively, so I went with the 90 synthetic (mobile 1) and the double capacity Mag Hytec diff cover.

 

You would also need 75W-140 if you are in what is considered a HOT climate.

 

He is right guys. The 75/140 gear oil is for 1500 model trucks. Our trucks use 75w90. Im not saying that the 140 weight oil will hurt it, but it is not what Dodge recommends. Look closely at the charts on this link>>>>http://dodgeram.info/tsb/2004/03-001-04A.htm Notice the gear oil weights for the various models and axles.

Anyway, for what its worth......I run 75w90 Mobil 1 in my axles and I tow heavy in HOT Texas heat pretty often. So far no problems in 78,000 miles. Currently I change out my fluids every 20k miles, but it is easy with the extra capacity diff covers with drain plugs.

 

I'm running 75w-140. No problems and no change in the fuel economy.

MikeyB

 

Amsoil makes a 75W-110 HDD full synthetic for those that want a bit more protection but don't need a full-on -140 oil.

I've had the 75W-110 HDD and a PML rear cover for about 5 months now and it has NOT affected mileage any. In fact, while driving in tight spaces (e.g. parking lot), less noticeable engagement of the limited slip, it has been a great rear end oil so far.

 

I don't like other folks puttin' stuff in my rear end.........................

 

I am running the Amsoil 75/110 and like it pretty well. SO far so good!