I have a number of customers who insist on using it in their cars and trucks, some use in the motor, some in the trans, and some in both. These are all older, high mileage vehicles, I would never recommend using it on newer or low mileage vehicles. I can't say anything bad about the stuff, there are some cars that it helps with engine knocks, and on cars with trans problems, it's a last ditch fix that may keep you going until you get a new trans or rebuild.
There are a cars where you really shouldn't use it at all in the motor, regardless of the age or mileage. Vehicles that use 0W20, 5W20, and some 5W30 shouldn't use it, especially those with oil pressurized timing chain tensioners. I've seen problems with these tensioners simply by using thicker 10W40 oil. Lucas seems pretty thick, so it may be a problem for these cars.