Just because an oil is thin does not mean its lubricating properties are substandard. In fact, John Deere tractors use Hy-Gard oil (which is like 5W-30) in the rear differential/transmission. This oil protects the ring gear and pinion as well as all the plantetary gears in the transmission. It has to be thin to have adequate flow for the hydraulic system, which uses the transmission/differential housing as a sump.
On tractors equipped with a mechanical front wheel drive axle (MFWD) 85W-140 was used in the differential and outboard planetary hubs. However, in cold weather research indicated that the heavy gear oil would be thrown out of the ring gear housing, leaving the spider gears without lubrication which dramatically shortened the life of the differential. Tractors tested using John Deere Hy-Gard oil operating in the same conditions showed no wear. Seal design improved and now John Deere recommends Hy-Gard in the outboard planetary hubs as well.
I feel this issue boils down to seal design more than anything. If your transmission, differential, ect can be fitted with modern seals, I would not hesitate to use thinner high quality oil. However, if you have a gearbox that leaks with gear oil presently, you won't be able to dump in the thinner oil fast enough.
