It's my understanding that the harder you work the truck, the more you need the higher viscosity lube.

 

Chevron makes two gears oils that are GL-5 rated at the correct spec'ed wieght for our axles. It is far cheaper than Mobil 1.

 

Tex,

The quote below is right out of my Service Repair Manual:

If you tow or haul heavy loads frequently, 75W-140 is probably the way to go. I do not tow heavy day in and day out so I elect to go with what the Repair Manual states and that is 75W-90 for my AAM 11.5. Also, since the front calls for 75W-90, it makes it easy just to have one weight class gear oil on the garage shelf.

Now I'll confuse you and let you know what AAM states on their site and that is 75W-140 for the 11.5.

Bottom line....either or, will perform well I'm sure.

 

I have been running Royal Purple 75w-90 for about 10K miles. My mileage increased about 1 MPG on average, but my truck feels like it rolls a lot easier when I push in the clutch. I have used RP products for about 5 years now and don't think I'll be switching anytime soon.

-Muzzy

 

Thanks Neil for the info but I am still a little confused.

I don't pull very heavy very often.......Maybe 6000lbs to 7000lbs once a month. I have pulled as much as 12,000lbs. for a several hundred mile trip.

If the 140 weight is better for pulling heavy.....then wouldn't it be better all the time? Is it alot more expensive than the 90 weight? Do I need to add an additive to the oil since I have the Limited Slip Differential? ( I am not sure what the "trac-rite" differential is)

Does Mobil 1 meet all the specs? IS there a Mobil 1 and also a Mobil 1 synthetic and does it matter which one I use?

Any info is very helpful. I just have no knowledge of axles and have NEVER changed the fluid in one on any vehicle before.

 

Here is my opinion. Everyone has one, right?

If you tow, use 75W140. Period. The heavier weight oil is typically priced the same as the 75W90. Mobil 1 is a GL5 synthetic and meets the spec although I am not sure if comes in the heavier weight.

Neil58, could you post the link on the AAM website that refers to the lube recommendations?

Casey

 

Amsoil says that their 75w-140 is "compatable with most limited-slip differentials." But they also have an additive if there is chatter. It also has a GL-5 rating.

Amsoil Severe gear 75W-140

 

I wouldn't say 140 is better than 90 but like I stated before, either or you can't go wrong. Only way I can expain this is that selecting gear oil is like selecting from a variety of different weight engine oil. Depending on your operating environment is the weight oil you choose. Africa Hot....you choose a heavier oil.....Artic cold, you choose a lighter grade oil. Same concept with gear oil. Towing/hauling heavy loads consistently generates quite a bit of heat back there in the differential. That is not the scientific explanation.....I'm sure one of our lube experts on the forum will chime in and give you a better expanation than I. Pick either one TEX, you can't go wrong and AAM states either or will work.



Mobil 1 is synthetic only! It does meet all specs and has been around with a proven track record since the 70's that I can remember.

Very easy to do. I would highly recommend buying a PML Cover that gives you extra fluid capacity, drain and fill pulg. This will make changing gear oil even easier and quicker. Otherwise, you will always have to remove differential cover on every lube change.......not a problem, just takes longer. Your cover has a reusable gasket too so you won't have to buy a new one each time you change the gear lube.

If I remember correctly, your first differential lube change comes at 15K miles....have to check the Manual to be sure......I changed mine before that to get all the particals out of there that come with breaking in a new truck......

Let me know if I can be of further help.

 

You guy's have provided me with lots to think about and plenty of good info, Mobil 1 it is. A number of my friends use it in their cars (engine oil) and swear by it

 

01350banshee - I don't think you can go wrong with the m1. Easy to find and priced reasonably... That's what's going back into mine for the third time in another 2k miles..

 

I used Castrol SynTech 75/90 when I changed mine. Hard to believe, but I simply can't find Mobil 1 gear oil around here. Someone asked about the downside of using 75/140. The downside is a slight decrease in fuel mileage. A thinner oil has less drag than a thicker oil. That's the primary reason the newer cars are specifying 5/30 or even 0/30 engine oil.

 

busted out the owners manual and it stated 75w-90 wt gear oil. I dont tow to much and when I do it's rarely over 7,500 lbs.

 

I second the AMSOIL SEVERE GEAR,I have the 75W 90 but next time I will go with the 140..

 

My '05 Hemi calls for 5w-20 motor oil. That is taking some getting used to but we tow alot and heavy and so far so good. I'll soon be ready to change out the rear end lube. Not sure if I'll go with 90 or 140. I like better gas mileage be it gas or diesel but I don't know how much of a difference it would make. One way to find out though...

 

I changed mine out a few days ago when I added my Mag-Hytec Rear Diff cover. I decided after reading this thread and some other consulting....and a little of my own intuition......to go with Mobil 1 75W-90. The owners manual does not even mention the higher 140 weight oil. I wonder if there is some differences in what the owners manual says for somebody with a Dana Axle prior to the AAM axle. Anyway, so far.....the MH looks good, is not leaking, and I presume the Mobil 1 is taking good care of my gears.