The approach seems logical, but has two flaws:

1. The assumption "Turkey is significantly bigger than Germany" only helps you estimate the market size, because the Lube Consumption per Population (the product of density and consumption) is the same value. If Germany and Turkey had a different value, making a simple assumption on size without executing the calculation doesn't work. When you prepare, prepare for the general case, not the special case that doesn't work in every situation.
2. The assumption is wrong. In the case, Germany is ~5M people larger. In reality, the difference has shrunk since the case was written, but Germany still has more people. When you make such assumptions, state them explicitly with the interviewer and confirm that it's OK to use them. If you just make them and power through to your conclusion, you'll get to the wrong answer without giving the interviewer the chance to help you course correct.