Should I change the oil?

I bought a used JD 950 Diesel about 6 months ago. The oil was last changed on February of 2008. The hours was also noted. Since the last oil change the tractor has 20 hours on it. However it has been over a year since the oil change. Should I change it or wait until it gets more hours? At how many hours or time should the oil be changed?

Thanks MTF for all the help you provide.

Tony