This is a simple method used by quite a few folks, but the problem arises that the tempering temperature and length of time held there, will vary depending on type of oil used and amount of oil used.
A LOT of muzzleloader Smith's use this method with good results!
However,
If one uses "too little" oil, the temperature is too low for the steel when the oil starts to burn.
"Too much" and the part might heat too hot.
"wrong oil" and we'll, your temperature is off.... It depends on the flashpoint temp of that oil....
If you wait until the oil burn off, that changes things
And of course "it depends" ™ on the steel used.
When making springs , consistency is important!
One of the more constant ways for tempering is to use the lead pot. And since we all cast lead bullets (don't we?) Everyone "should" have a lead pot and a lead thermometer....
Soaking the the springs in the molten lead is very consistent, if one knows the melting point of their alloy, and uses the thermometer to keep the temp there....
Of course, this is from the guy who keeps breaking springs, so, take it that into account....
Yhs
Prof spring breaker