My recently-acquired Tundra 4X4 has never had the gear oil changed in the differentials or transfer case. The manual calls for API GL5 75W-90 gear oil. However, a guy at a very reputable high-performance 4X4 shop said he puts ATF in a lot of differentials. Subsequent Googling has indicated that others do this, too. But it doesn't seem widespread. He said 75W-90 was "old school." As I am going to do this work myself, I am trying to decide whether to just go with what the book calls for or use ATF. Will ATF be harmful if I change it every 30k mi? Will there be any benefits in terms of economy, performance or MPG? Thanks.