As far as emissions go, any old mechanically injected diesel is a polluter. Diesel fuel is just oil, just thinner than motor oil. If emissions was the issue here, I guess we'd all have to park our 4.3s, 5.7s, 6.2s, 6.5s, 6.9s, 7.3s. 5.9s. 1.6s, 1.8s, 1.9s, 2.2s, 2.3s, etc.
How many emissions are created from fueling and tooling what's needed to make new cars and trucks, and recycle crushed cars and trucks, every year? The answer is, a lot. So, in some ways, keeping an old diesel running regardless of fuel used, has a positive note to it.
As far as legal goes, that is different state to state. Federal law allows an individual to use "home brew" fuel of any type up to a certain limit of gallons, per year, on public highways. But, some states do NOT allow it without payment of taxes. Also, there is a legal allowance up to a certain amount of "additive chemicals" put into the tank, per person, per year.