I've used soy/peanut/mixed veggie oil from time to time up to 50% on all of my
300tdis that I've had, it honestly runs better but Dave's right, it needs to be warm out. I've also used mixes of B20 to B100, same results, IMO it runs better. With used oil (motor/gear) I'd make sure its
filtered well before it goes in your tank, you don't want metal bits in your fuel system. I keep an eye on FB marketplace and other sites for people giving away free/expired motor/2-cycle/veggie oil and have gotten free fuel several times. - have a spare fuel filter on hand...