changing oil on rear axle on 2002 3500, 2wd. question about LS additive

[Motorhead]

I am doing a service on a neighbors '02 3500 dually. It has limited slip. When I had my 2001.5 with the limited slip, I was told at the time if I switched over to full synthetic that I didn't need the special LS additive from Chrysler. I changed mine over to full synthetic gear oil and don't remember that I had any issues without the LS additive. I sold the truck when I bought my 2007, 5.9 third gen.
So what's the consensus? Do you omit the additive when using full synthetic? The lady that owns the truck pulls a semi large horse trailer with living quarters. Would the recommendation be to go with the 80w-90 or 75w-110? Thanks for any help.

 

[DavidC]

Mine has Royal Purple 75-140. No additive

 

[Motorhead]

Thanks DavidC!

 

[KATOOM]

Most synthetics purchased at parts stores are going to contain friction modifier already in the recipe. Conventional fluids will not. Install and if the differential chatters during turns when its at operating temperature then you'll just need to add some.

The viscosity weight is best determined by the average ambient weather where the truck is driven and what its used for. 75-90 is the most common unless you're towing heavy in triple digit weather, then 75-110 or 75-140 can be better. But if you live where it gets frigid cold then I wouldnt go any heavier than 75/80-90.

 

[cerberusiam]

With an 02 it DOES need the additive. That is a clutch type LS and will raise all manner of havoc if it isn't added. The GL-5 fluids do not contain the correct additive for a clutch type LS, only for the Torenson types.

 

[Motorhead]

Thanks Cerb. So my feeling is, if I need to put the additive in, I can save my neighbor the extra money by buying conventional petroleum based oil plus the additive instead of the synthetic. She is on fixed income, 75 yrs old and still rides her horse everyday. She is taking more LOCAL trips now than the 6-8 week trips to Utah and AZ where she went and explored all over with her Gelding, Smarty. Remarkable lady!

 

[NIsaacs]

Tractor supply travelers 80/90 has the LS in it. I use the same stuff in my trucks in 85/140.

https://www.tractorsupply.com/tsc/p...rpose-gear-lubricant-80w-90-1-qt?cm_vc=-10005

 

[cerberusiam]

Yeah, all I have used for years was 75/90 or 80/90 with 2 containers of the Mopar additive. One did not do it on a good LS, always needed 2. The only diffs that ever worked with one bottle of additive or the fluid with advertised additive were the wore out ones.