Hmnnn. Looks like I may have gotten some bad info from my
local Dodge dealer, but anyhow I'll add my 0. 02 here.
My truck is 05 2500 4x4 QC short bed.
At about 12K mi, I installed a mag-hytec rear diff cover and
chose to change rear diff oil in the process. I used the dealer's
synthetic ($$$) fluid largely with warranty in mind. Dealer told
me I yes needed to use the limited slip additive, so at first
purchase I bought one 4 oz bottle of limited slip.
Since new, the truck drove fine but I could notice some rear
end shudder when turning tight corners such as turn from
stop sign etc. Not of major concern, reminiscent of the old
muscle car days driving a muscle car with posi rear end etc.
I drained fluid, installed mag-hytec cover. Noted about zero
wear, no shavings, just a very small amt of metal powder = good.
I refilled to same as stock oil level. (while I had both covers off,
I matched up the bolt holes and sighted the bottom of both
cover plugs were at same vertical level). On the initial fill I added
the single 4 oz bottle of limited slip additive. I think the total
was 5 qts diff oil + 4oz bottle limited slip + just a bit of 6th bottle of oil.
Side note: According to the instructions on the limited slip additive,
the amt I had installed at that point was a little on the low side
of percentage limited slip for the given amt of rear diff oil.
The truck drove exactly same as new. No major issues, but noticed
some shudder on sharp turns.
At a later date (couple weeks), I purchased second bottle of
limited slip additive. I added about half (2 oz) of that bottle, and
at same time decided to add rest of the 6th qt of diff fluid.
(this brings rear diff oil to slightly higher level than stock fill plug
but still well below the level on the mag hytec dipstick. ie. above
the lower parts of the axle tubes, but not nearly up to the axle shafts)
After adding the extra limited slip + remaining portion of the 6th
quart (I estimate it was about 80% of the 6th quart I added)
I notice no longer shudder on tight turns.
I guess we can't tell whether the shudder disappeared because
I added 2 oz more limited slip, or because I added about 3/4 of
another quart of diff oil.
From reading earlier parts of this post, looks like my dealer may
have given me bad info and I didn't need to add the limited slip
stuff at all. Just thought I'd offer my experience.
BTW also installed temp gauge dedicated to rear diff, using
temp probe location in the mag-hytec cover. That was perhaps
the biggest surprise of all... In upper 90's TX sun, towing
an enclosed flat front Haulmark relatively light at about 5000 lbs total,
65MPH steady hwy, rear end ran about 195F. I didn't realize
they got that warm. Have since been told by others that's normal.
Runs 140-150F hwy on similar day when towing nothing.
Attention: TDR Forum Junkies 
RP 
