Food for thought, thanks, cloddopper!
Especially so because I now know that multi-fuel "camping" stoves are not all equal. Was starting to wonder whether I should get one that does not require a fuel-jet change-over for different fuels. There are a few designs that can burn any liquid fuel without swapping jets: a "one-jet" design should surely overcome any quandary over the jet to use for 2-stroke fuel. As might be expected though, there is a price differential!
No panic yet - my 5 ltr waste-fuel container is far from brimming.
I shall think about your comments/investigate: while I know that used-oil recyclers are not happy if coolant has been mixed into the oil (it happens frequently apparently), I had not considered that old-oil re-cycling processes can tolerate a bit of fuel in the mix!