When talking about diffs here is what you need to know, keep in mind i dont know everything about each weight
A). The thicker the weight the more resistance so the the more power it takes to turn it. I run K in both diffs. Works really well for all around bashing, mild and heavy.
Shock oil
A). Thinner oil the less pressure it takes to push through it. A thin shock oil will flow through the piston much fater giving you less resistence and a softer suspension. Its like 15wt is water, and 70wt is pudding. I run 55Wt in U.E shocks.