I’ve forged a Kukri from a blade from a John Deere mower. Does anyone know the right quench liquid for this? I have plenty of oil and water available, but I want to make sure I get it right. Any input is appreciated. 

John Deere says their lawnmower blades are made from spring steel. On that basis, if you proceed as for 5160, you'll probably be okay. 

You don't say what kind of oil you've got, but I've personally had good luck with canola.

Burnt motor oil 

Used motor oil has many nasty things in it. There is a post on the site about why it should be avoided.

Besides if the motor oil were burnt, what would be left, soot and some ash?

I've done heavy mower blades in canola oil before and they turned out ok. A gallon of it is pretty cheap at the big box stores. It's cleaner and smells a lot better than motor oil.

You could make friends with someone at the local deli etc that uses a deep fryer and they’d probably let you have their old oil. It may be a little fishy smelling but it’s free and much better (for your health) than using old motor oil.

Or seek out a doughnut shop. 

Less fishy. 

I concur on Canola oil. I pick up big gallon jugs for $6. I have used motor oil, and I ‘now some guys use it, but after reading the threads about motor oil that Glenn mentioned, I felt a six dollar investment was worthwhile. I have been happy so far