JD 310D 1991, foam oil in Hydraulic Reservoir

Hello my friends, first, thank you for allowing me to enter this community, I'm Southbus, i live in Chile a long and flat country in Southamerica, i have a old but strong John Deere 310D 4x4 Backhoe.

The past week a lot of oil foam go out from the reservoir when uncover to inspect hydraulic oil level,
Large air bubbles emerge from the bottom of the tank, after about 10 minutes the level of the foam decreases but the oil stay with gray colour, i think millions of small bubbles are mixed...

JD 310D 1991, foam oil in Hydraulic Reservoir

JD 310D 1991, foam oil in Hydraulic Reservoir