I suggest you change it. Motor oil used in hydraulic applications is subjected to constant "shear" as it goes through pumps and motors, and is by-passed and forced through tiny orifices. It tends to break down the molecular structure of the oil, which isn't visible to the naked eye, and would only show up in an oil analysis. There's really no way of knowing exactly when it needs changed, short of paying to have an oil analysis done...
For a predominantly warm climate, where you don't have to worry about cold weather starting, I would certainly recommend 15w-50 instead of 10w-40. The thicker the oil (the higher the viscosity), the more it will resist shearing and by-passing, and the more hydraulic power it will transfer -- within reason of course, you don't want to try pumping gear oil through a hydraulic system.
IMO, you'd likely notice the difference in power transferred to the wheels, for example, after a fresh oil change -- especially if you step up to the 15w-50. I certainly did, as did others who changed to Mobil 1 15w50, for example...
The biggest reason to change it is that the oil is absolutely full of suspended metal particles that are small enough to pass through the 10 micron filter on the PT hydraulic system -- not to mention the fact that that filter doesn't filter all the oil that is going through the pumps all the time. Much of the oil that goes through the pumps (for example PTO pump when not using the PTO) gets dumped right back into the reservoir without going through the filter.
When you get the used oil out, you'll be able to see these tiny suspended particles of metal reflect light and you'll likely be surprised how much metal was floating around in there. When I changed the oil in mine, I put the used oil right back into the one gallon jugs the new oil came in. A few months later, I looked at it, and there was a grey sludge of metal particles that had settled out of the oil on the bottom of the oil jugs that was 1/32 - 1/16 inch thick, I'm guessing. And this was from a machine with about 300 hours on it at the time...
My 2 cents, your mileage may vary...