Introduction:
For the UPSC Law Optional subject, it is absolutely permissible and even recommended to refer to legal commentaries and expert opinions. These resources can significantly enhance your understanding of the subject and provide valuable insights into complex legal concepts and principles. However, it is important to use these resources judiciously and ensure that they supplement your own study rather than replace it.

Benefits of referring to legal commentaries and expert opinions:
1. Enhanced understanding: Legal commentaries and expert opinions offer in-depth analysis and interpretation of legal provisions, cases, and principles. They provide valuable explanations and insights that can help you grasp complex concepts more effectively.

2. Clarification of doubts: Studying legal commentaries and expert opinions can help clarify any doubts or confusion you may have regarding specific legal issues. These resources often address common misconceptions and provide clarity on complex legal matters.

3. Exposure to different perspectives: Legal commentaries and expert opinions are often authored by renowned legal scholars and practitioners who offer diverse perspectives on legal subjects. This exposure to different viewpoints can broaden your understanding of the subject and enable you to develop a more comprehensive approach to legal analysis.

4. Updated information: Legal commentaries and expert opinions are often updated to reflect recent developments in the field of law. By referring to these resources, you can stay abreast of the latest legal interpretations, judgments, and amendments, which is crucial for the UPSC examination.

5. Case law analysis: Legal commentaries often provide detailed analysis of important case laws, highlighting their significance and application in different scenarios. This can be particularly helpful in understanding how legal principles are applied in practice.

Caution and guidelines:
1. Supplement, not replace: While legal commentaries and expert opinions are valuable resources, they should not replace your own study and understanding of the subject. Use them as supplements to your primary study material.

2. Reliable sources: Ensure that the legal commentaries and expert opinions you refer to are from reputable sources and authors. This will ensure the accuracy and reliability of the information you gather.

3. Multiple sources: It is advisable to refer to multiple legal commentaries and expert opinions to gain a comprehensive understanding of a particular legal issue. This will help you develop a well-rounded perspective and avoid relying on a single interpretation.

4. Balance with other study materials: While legal commentaries and expert opinions are helpful, it is important to strike a balance by referring to other study materials such as textbooks, bare acts, and case law compilations. This will ensure a holistic preparation for the UPSC examination.

In conclusion, referring to legal commentaries and expert opinions is highly recommended for the UPSC Law Optional subject. These resources can enhance your understanding, provide valuable insights, and keep you updated with the latest legal developments. However, it is important to use them judiciously, supplement your own study, and maintain a balanced approach by referring to multiple sources.