What type of information should the vacuum gauge display?

The vacuum gauge is specifically designed to measure the vacuum pressure present in the engine intake manifold. This information helps in assessing the engine's performance, tuning, and troubleshooting various conditions, particularly related to the brake system. When it comes to the braking system, a vacuum gauge can provide insights into brake pressure and the efficiency of the brake booster, which relies on engine vacuum to function properly.

Understanding this context is crucial because it allows drivers or mechanics to determine if there are any issues with the braking system or whether the engine is creating sufficient vacuum for optimal brake performance. The other options, such as engine oil level, battery level, and transmission fluid temperature, are monitored by different gauges or indicators and do not relate to the function of a vacuum gauge. Each of those measurements is critical for maintaining vehicle health, but they fall outside the vacuum gauge's specific operational scope.