The thicker silicone that you use the easier it is for your truck to come to a rolling stop w/o using the breaks. (freewheeling)
Thinner silicone will allow your truck to roll easier, thicker silicone will give more resistance.
I may be wrong but I would think that when the truck is rolling straight ahead that the internal diff parts (the ones affected by the oil) wouldn't be turning at all, so the diff oil wouldn't affect stopping.