I suggest Rotella 15W40 or any other high quality 15W40 or 20W50 oil. 20W40 is very hard to find and costs around $10 a quart at an Ingersoll dealer. I also recommend that you have a large drip pan and lots of rags when you change the oil because I created a HUGE mess when I changed the oil in my 446. Admittedly it was my first time changing the hydraulic but I was somewhat surprised at how messy things got. After removing the drain plug on the bottom of the TCV, I ran the motor briefly with the TCV in the ahead position to speed-up the oil drain. It "drained" plenty fast (more like sprayed out the drain hole)! Fun experience but again, a little on the messy side.
I cannot think of a single valid reason to not use 5W40 oil in either synthetic or conventional. The edge would go to the synthetic because it would likely make starting the engine in really cold weather a bit easier,