I'm sorry for giving the impression that I'm already doing this. Re-reading my post, it can definitely be taken that way. I haven't done it yet, just at the "looks doable on paper" stage.
I would likely filter via settling, and gravity feed through a toilet paper type filter. I probably wouldn't de-water at all.
The whole process would have to largely be set and forget, and not burn through expensive filters. Going to the recycling center is a pain, but not hugely out of my way if I route my trips right. How much WMO can be filtered through a filter before it needs replacing, and how much diesel will the cost of that filter buy? Doesn't take much time or cost consumed before I'd be better off just taking the oil to the recycling center and buying diesel.
denim works well as a cheap pre filter, takes the oil down to 15 micro if I remember
But after the denim you're going to want two of these 10 inch filter housings , a 10 micron then a 5
, you can either gravity feed or pump it through
After filtering you will need to get the water out some way or another (otherwise you'll knacker your engine) Buy a water block filter, or make one using Fuller's earth cat litter .. (you can reuse the cat litter over and over, just dry it out in the sun) . Remember oil floats on water , so your cat litter wants to be low in your homemade water block filter
You're other option is to by a small centrifuge, they're cheap enough, that avoids filtering & dewatering
It needs thinning with some form of solvent, kero, white spirit , petrol ( don't use anything that dissolves rubber seals like acetone),
normal unleaded petrol is best because it's guaranteed engine safe ( or gas if youre American)
5% 10% 20% depends on your engine & how cold it is there , start with 5% build up, you'll have to play with it
Black diesel is great for diesel engines, they love it. Get it right and it'll run great