TractorData.com Massey Ferguson 165 tractor information
Your hyd system is open center, and the pump flow is 4.5 GPM
If you have remotes, and the two hoses are connected to the remotes, then you would plug in the log splitter and bungee the lever to supply fluid to the log splitter.
If the log splitter valve does not have PB, then you might want to run the OUT hose direct to tank.
Some tractors use the PB out from the FEL valve to supply fluid to the back. When the log splitter is not connected, the PB hose is looped to either the remote IN port, or the 3pt IN port, and this will complete the series connection flow path.
The log splitter has it's own valve. The log splitter OUT port would go to tank.
If you would show some pictures and further explain exactly what you have, we can help you more.
You would set your log splitter relief the same as the FEL valve relief.
So if your tractor has a pressure of 2750 psi and your log splitter has a 4 in cyl and 2 in shaft, you would have a push with a force of 34,557 lbs, or 17.25 tons.