Why are older diesels rainforest killers? Is this an indirect refernence to global warming due to Co2 emissions? Does smoke kill rainforests?

An old diesel running off of waste vegetable oil or biodiesel is largely carbon neutral. Can't do that with your gas machine, less you run it on biologically produced butanol, which isn't readily available - yet.

Its interesting that the EGR in these older vehicles can be the source of the excessive smoke problem.