I don't know about the cetane of old engine oil, but as for plant oils, the cetane and other properties are listed here:

vegie oil table

I generally use Powerservice Diesel additive with every fill up. It noticably quiets the idle knock, so in terms of idle quiteness, I think cetane matters. As far as power production and effeciency, you would probably have to retune the injection advance to get any benefit from higher cetane. I don't see any real fuel economy difference with or without the additive.

Sholin

__________________
What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.