I believe fuel nozzles and refuelling ports were 're-engineered' so that only the correct nozzle would go into the fuel tank.
That was a suggestion only, however the onus was on the aircraft owner to do it. The same with grade decals - it could be suggested to the aircraft owner that they apply them but that's all. BP had a policy whereby if no grade decals were present then the pilot had to fill in a fuel order form before the refuelling could take place.
DF.