Ok, so how do I make my oil in commercial deep fryer last longer? I would be willing to filter it twice a wk. although not twice a day evrery day. What other things do I need to know? Are some oils better than others and why? Smoke point or viscosity, impartment of flavor? Do people use lard and oil? Hey can I still use tallow?/img/vbsmilies/smilies/biggrin.gif lol. I change the oil every 10 days, so I go through 60-70 liters a mnth. I should have bought a diesel car. Am looking to extend oil life up to 21 days. Any suggestions would be helpful.