I would think gear oil would be the way to go, but if someone was real concerned then why not just thin it down a little by mixing some hydraulic oil with the gear oil? It's all petroleum based, so it should mix fine, right?. I had an old d4 high lift a while back that leaked gear oil out of the final drive pretty bad, so I mixed hard oil (grease) with gear oil and put it in there. It slowed the leak alot.