Totally depends on;
your style of driving
your truck's setup
track setup (assuming you're racing)
track surface
what you want to accomplish with the truck
and on and on...
As a brief example, in a typical situation, the thicker the oil in the front, the more the front end will push in a turn, or
under-steer (the thicker the oil in the rear, the more the truck will typically over-steer).
However, on a choppy track, or a track with a rhythm section, thicker oil in the front can cause the truck to
over-steer during uneven suspension compressions. Also, just changing tires can alter diff requirements greatly. Using 30-10k on a loose surface with ABC tires, can yield a very similar result to using 10-5k on the same surface with XYZ tires, etc.. So, unless the person advising you on diff oil selection is running at the same place with the same driving style and the same steering/suspension geometry setup, with the same tires, etc., the advice is going to be a hit and/or miss. You've gotta make note of what the truck is doing and what you would like it to do differently, and make decisions based on that information. There's really no magic setup for all situations.
