Those older grades are suggested for their higher Zinc additives compared to SL or higher grades. Zinc compounds protect and lubricate under extreme pressure condition. But they tend to damage cat cons, hence their usage and popularity has declined throughout newer grades.
Diesel oils still have lots of zinc additives, lot more than any old SG, SJ. They also have tend to withstand higher demanding conditions without changing their chemistry or breaking down, have better foaming control, have better oxidation control, better cleaning properties... and though not related to transmission: very very high reserve alkalinity.
And last but not least: they are some times even cheaper than inferior petrol grade oils. WIN WIN.