emissions wise its a bad idea
even if you filter it through a 1 micron run, its still full of unburnt fuel, crud and additives, these additives are not designed to be burnt in a engine and produce bad stuff out the tailpipe, not smoke, but things we cant see like NOX, and sulphur dioxide,
I would shy away from burning it, let it get recycled and used again as a motor oil
you do know that Recycled motor oil has to meet the same specs and designations that new virgin oil does to be sold as motor oil , the oil itslef doesnt wear out, its the additives package that gets used up
so it gets filtered and re-additive-ized (new word) and sold as recycled, but its just as good as virgin oil.
and one more tree hugging thought
in most states if a retailer sells you oil...... they must take it back to be recyled.... free of charge