On my L185 it was a seal failure on the hydraulic pump power input shaft that fed hydraulic fluid into the engine crankcase.
Take pump off engine, replace seal, install pump, pour hydraulic fluid into the top opening of pump before you install the pipe. Change engine oil and start tractor. Done. Check engine oil obsessively for next few weeks until you’re sure that you have it fixed.
If the tractor is older and you’re not positive that the pump is original to tractor and there are serial number breaks for the seal, I suggest you order all the seal part numbers to be sure you get one that fits.