Modern diesel pumps are very different things to kind fitted to older cars. A common rail diesel engine can easily run at fuel pressures of 30,000PSI.
Exactly what he says.
Older diesels would if anything, benefit from a bit of petrol being chucked in with the mix (not too much mind). Petrol is a thinner, it helps thin the diesel slightly.
Modern diesel pumps run at ridiculous pressures though, and petrol in the mix is an absolute no go. It's also why you can run older non high pressure diesels (like the Pre-PD diesel in my VW Passat) on bio-diesel (or at least a mix of it and normal petro-diesel), whereas on a new common rail, high pressure diesel engine, you can't! You've got to run em on neat petro-diesel…
So whilst newer diesels generally produce higher specific outputs, and less CO2 per KM than an older diesel, they're potentially less environmentally friendly as you can in theory run older diesels entirely on someone else's waste products…