Was changing the oil in my gearbox the other day & it got me thinking... now I'm the first person to say used engine oil is a bad idea for bar lube for a number of reasons, but most of the reasons that come to mind for engine oil aren't (as far as I'm aware) nearly as relevant for the likes of gear oil (diff, gearbox, lathe, etc) or non combustion motors like compressors etc. Asides from the lack of tackifier, are there any significant reasons not to filter these & either add them in low ratio to regular bar oil or use them in the aux oiler for the mill?