OK I know I will be called naughty name by some and not overtly bright by others. In MHO unless the oil has been contaminated but something, it never wears out, really oil never wears out, the additives do yes. When I have done the hydro I pull the plug, and drain the oil through several filters in to a CLEAN container. And reuse it, adding new to bring it level up to what I lost.
Is this the best or even a good way to do it probbly not, just one mans opinion.
Now on a big tractor or large piece of equipment that works around dusty or dirty conditions everything gets replaced due to contamination