Somewhere in the systems you are having energy losses.  These losses are generating heat.

What are the purposes of the hydraulic systems?

A quick one to check is a relief valve that is set too low.
Or, somewhere there is a flow restriction creating a lost-energy pressure drop.  Pinched hose.  Pinched tubing.  Hydraulic lines too small for the flow rates.

Are there any heatexcahgers in the systems?

A circuit diagraphm would be helpful.

What changes or adjustments have been made to the systems since they were first set up?

Has heat been a problem since the systems were first set up?

Ted

Depending on what you are doing with the hydraulic system you get varying amounts of heat into the oil/fluid. A properly designed system will have equipment to get rid of excess heat such as an oil cooler or a properly sized reservoir. In some applications the settings of the relief valve may impact the amount of heat put into the oil. Ambient air temperature has an impact also.

There are some applications such as continuous testing of two hydraulic cylinders working against each other all the energy of the work is absorbed by the oil so a massive cooler could be required in this application. Your application is having intermittent overheating problems so if the application is consistent you should either shut the process down during times of high heat or install an oil cooler in the return to tank line. Wherever you put the oil cooler in the system check the pressure in the line and make sure it does not exceed the oil cooler max pressure rating. The size of the cooler will depend on how much excess heat you are seeing the the cost impact of changing the seals.

if O-rings and seals are damged by the high oil temperature the oil temperature certainly is too high. have you found any relationship between the oil temperature and the ambient temperature and or the type of load on the system? if there is a relationship, a properly dimensioned cooler can solve the problem. if there seems no relation at all, the actual system should be investigated to see whether all the valves in the system are working as they should and especially whether they are closed when they should and not "nearly closed". in the latter case you might be pumping around oil through very narrow orifices which is a certainly a cause for overheating. usually a valve that is not working properly can be heard: it produces a "screaming/whining" sound. another item to look into is oil viscosity. both too high and too low a viscosity might lead to higher temperatures - what viscosity is used in the system and what is the ambient and oil temperature?

Have you ever changed the hydraulic oil, is there a preventive maintenance program?

Your description is too vague.What seals are being damaged?At the relief,the whole system?Do you run oil coolers?A cooling control valve(thermostat)?Working components?Describe your system in detail along with all info on problem ,supply hyd.schematic?And an educated logical approach can be given.BTW it sounds like your engineers need training