I remember a "joke" if you could call it a joke about a cardiothoracic surgeon taking his motorcycle to the mechanic and the mechanic asked the doctor why he made so much less money and the doctor replied that he had to operate on a person's heart while it was going (he didn't, as the heart-lung bypass machine takes over the heart's pumping function while the surgeon operates). Granted, you don't need as much training to be able to replace an inlet valve on a car's or motorbike's engine as you do to replace an aortic valve, what you have to remember is, the cardiothoracic surgeon won't be able to tell you what's wrong with your car, while a mechanic can't perform heart surgery. And, if a person needs a double bypass and a heart valve replacement, the cardiothoracic surgeon replaces the valve and a vascular surgeon takes the artery and grafts it to the coronary arteries.

Is a doctor more knowledgeable than a mechanic? About the human body, yes. About car engines, no. Well, if their father was a doctor and bought them a brand new BMW for their 18th Birthday, probably not, but if they grew up in a working class family, in the 1980s and their father bought them a secondhand Holden Gemini or Ford Cortina for their 18th Birthday, they might have changed the oil themselves during the end of semester holidays.