That sounds like some pretty thick oil for the diffs. I have a 7.5 and run 5000 front, 7000 center, 1000 rear and that combo works excellant for most loose dirt tracks big or small. I would say using the oil you described would give very bad handling, pretty much eliminating the diff action all together. Give us more information on the type of car you have and the type of track you run on for better info.