I was wondering what type of oil I should use on an old car leaf spring. Right now all I got is old motor oil I have sitting around would that work or no?

A lot of the old springs are made of 5160 steel.  Assuming yours is one of those then the short answer is yes it will work.  The longer answer is that used motor oil is not recommended.  It's not because of the quenching properties of the oil; it's because used motor oil tends to contain a lot of stuff that isn't good for you to breathe or absorb through your skin.  If you decide to do it anyway please quench outside, wear appropriate PPE, and/or avoid breathing the vapors that result from quenching.  In general for best results with most oils you want to warm it up to around 140 degrees F so that it flows well and provides a better quench.

I don't know where in the world you are; but here in the USA we generally advise quenching in warm cooking oil---and you can use old fry oil with no problem. (got a friend in fast food that can get you a gallon or two when they clean out the fryer?  Know someone that deep fries a turkey 1 or 2 times a year and then just discards the oil?  or even gasp buy it new---some places even carry it in a 5 gallon container which is way more than you need for *most* things...)

Ok thanks for the info I will try to get my hands on some fry oil when I can