Heh, the VW didn't "figure out" it had diesel instead of gas in the tank... it just couldn't run on gas. On a 2009+ TDI filling the tank with some gas will make the high pressure fuel pump come apart and fill the entire fuel system with metal shavings. The normal dealer procedure is to replace every single component fuel touches, to the tune of $6000+
Leaded gasoline and auto diesel fuel nozzles are the same. High rate truck nozzles are bigger. 2013+ VWs have a device in the fuel filler neck that prevents the smaller unleaded nozzle from going in!
There are no 1988 Cadillac diesels... unless someone was swapping things around. I've seen swaps where a 6.2 or 6.5 is plug in place of the 5.7 gas a Broham would have come with. 1985 was the last year GM sold their "classic" IDI diesel passenger cars, with the 5.7 V8, 4.3 V8, or 4.3 V6. None of the old GM IDI diesels were very powerful, though they could get acceptable fuel economy. A 6.5 TD mounted in a car would probably give acceptable performance, and with the 4L80e's lockup torque converter you might be able to touch 30 mpg!
-J