In a perfect world I think that either grease or gear oil would work, but we don't live in a perect world. The whole gear oil idea only works if the seal is intact. What happens if you driving down just a two track and a stick or sharp rock comes in contact with the rubber/felt seal and compromises the seal. Bye bye gear oil and your running dry. Grease will stay in there and get you home. Gear oil will be gone and leave you standed. Maybe the Toyota engineers aren't so dumb and thought of this when recommending grease in the birfs, knuckless, and wheel bearings.

