I don't..
The guy that does fish and chips down in the valley is really particular about his oil and changes it frequently. I pick it up in clear plastic containers and it looks like new,I run it through a filter, made of a couple of layers of old cotton sheet and that's it.
If there's some around I add a bit of sump oil when I pour it into the tractor, a 60's Mccormick, and that's it.
It takes a bit of cranking into life when it's cold, the battery is not the best, other than that it runs well and smells great when it's hauling a 4 ton trailer load up the steeper forest tracks.
I intend to run the Lister on it eventually, to which end I'm half way through building a crude mechanical power meter and a dummy load cell. If I discover anything meaningful I'll post up.
Cheers Stef