Differential oil change questions

[HomerDog]

Guys,
I'm changing my diff oil on a 96 3500 4X4 with a Dana 80 limited slip. I'm going to put in Mobil One 90w sythetic. I don't know what was in there before I bought the truck, but I'm assuming non-synthetic. It has 137K. Questions are? Is it wise to go to sythetic at this mileage, and if I do, do I have to add the friction modifier like the service manual says? Aslo owners manual claims a 3. 3L capacity while the service manual says 4. 8L (0. 3 being friction modifier). How much do I put in? Last question, what about sythetic engine oil at this mileage? Thanks in advance,

Dave

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'96 3500 Dually 4X4, Black, 4 inch exhaust (no Kitty), chrome kit, Rancho 9000 shocks, '00 Lance camper

 

[LSMITH]

Synthetics are not going to hurt anything at your mileage. You might find some leaks but I doubt it. I recently put Mobil 1 in the diffs and no modifier, the bottle said it had it. I have not had any chattering in the LSD yet. I have about 2500 miles on it now with the Mobil. I would not hesitate to put it in at 140K.

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99 2500 QC 4X4 AUTO SB 24V LARAMIE SLT 3. 5 LSD 285-75-16'S SPA Boost/EGT Rancho 9000's Everything but leather NRA Member Great Lakes TDR Member

 

[Cooker]

No problem putting it in at that mileage. Mobil 1 does have an LSD additive but you may find you will need to add some more, maybe not.

-Ryan