So, where I live petro diesel is $4.79/gallon, B100 is $5.00/gallon and old fashioned kerosene is only $4.09/gallon. Highway tax issues aside, how much kerosene is too much to blend in with regular petro diesel to run my '86 190d 2.5 5spd?

I've heard some say 1/3 kerosene is safe, and others say you can mix 50/50 kerosene and petro diesel, but only to do that in the winter months. I live in God's country (the South) where summer temps average high 80's and low 90's from June-September - how much can I mix in without fear of heat and lubrication issues?