Industrial hyd tech here. A properly running system shouldn’t get that hot, even with regular cyclic use all day. It’s not so much the high pressure that causes heat, but the pressure differential when going from high to low pressure (without doing useful work) that causes heat to build. There should be a relief valve on the system that is used to protect the pump and/or cylinder against spikes and if it’s improperly set or faulty can cause heat. Internal bypassing of the pump can cause excess heat too. As long as your not running in the direct sunshine on a hot summer day, heat shouldn’t be an issue.
Make sure you pause in neutral on your spool valve when going from extend to retract too. Shifting too quick can cause pressure intensification that will blow seals and pumps quick, and can also cause heat.
Yes a larger reservoir helps, as will a conditioning loop or heat exchanger, but heat shouldn’t really be a problem if it’s set up correctly. When I build an hpu, I try to make sure the reservoir is at least 66% larger than the capacity of the rest of the system. Also, avoid streamlining the fluid in the reservoir, where the pickup and tank line are close together or pointed at each other.
There’s a lot of good info and knowledge from the previous posters. If your splitter is a commercial unit, it should be designed correctly (I have yet to see one that isn’t). The home made ones though, they are often very incorrect in their design.
If anyone needs help troubleshooting or dialing in your system, shoot me a pm and I can give you some pointers.