When I owned a BJ42 landcruiser (very simple 4 cylinder diesel), I used to dispose of old cooking oil by running it through some flannel shirst material and tipping it into the tank before fill up.
If I still owned it, I'd esterify the stuff first, but I didn't know about that then.
There's a lot of good information on the net. I can post some links if you want.
Now the old oil gets turned into soap.