

The problem, at least in the UK is exactly that. As commercial vehicles, they can be more tax efficient than company cars. Lots of builders and mechanics have them as family cars, because this avoids the traditional emissions based personal company car tax system. I appreciate this is more of a problem with the UK commercial taxation system than anything else but still
You could also convert cars by putting harder valve seats in, not sure if this happened a lot at the time, or whether it’s more for classic cars that are worth enough to make sense to do. I don’t think the lead was primarily an anti knock additive, more of a valve and bore lubricant