Ultra fine sawdust, not the chipped stuff. You will need to visit a real sawmill or horse stables supply shop, as it is the very fine darker brown stuff that works best. I use a hair dryer to get the moisture out, then mix it with the gearbox oil before use and change the oil every year. If you want it to last longer then soak it with some gearbox oil Moly before mixing with the box oil. Works wonders in a classic tractor box.
I can't think anyone uses oil thickner anymore as Mobil 10/60 EP would be better in the engine. Don't put sawdust in the engine oil or an auto box, just old tractor boxes that are rattling and leaking their way close to death or you will block the filter.
One teacup for 10 litres, mixed with a half can of gearbox Moly.