You can add used hydraulic oil as well as other used oils.
You could also check with local transmission shops and mix used ATF.
Ideally, it needs to sit for a week or so to settle out solids and then get a good filtering (multiple passes).
Your filter will clog faster with adding these used fuels, so keep a spare or 2 with you at all times.
A sedimenter will go a long way to trap water and particulate.
I had some good quality ones manufactured, so if you don't have one already, PM me for details.
I'll put the FS post up again in case you didn't see it.
Uncle "Richard" Douglas has a Land Rover with big wheels that never gets stuck... until he breaks something so it won't go. Uncle Douglas always breaks something. - Anna Crowther at the Conclave 2012 (AKA Carburetor Neck)