DK35 Instrument light

I am trying to find out what a light is and why it comes on after 30 - 60 minutes of driving. The light is on the bottom of the instrument pannel about 2 inches off center to the right. It is a square box with a line in the center that has 1 drop in the top and 1 drop in the bottom. All my fluids are full and it does not appear to be overheating. Thanks....

According to my service manual it looks like it could be the hydraulic filter warning lamp.
You may have a plugged filter.

thanks

I changed the filter but it still has the same issue. The light comes on after about 1 hour of driving. Anyone have any other ideas that may be causing the problem?