You MUST check your owner's manual. This is the single best source of information. It will specify the correct oil type (straight weight, multi-grade) and viscosity (e.g., 10W-30, 20W-40) for your specific engine.
If you don't have the owner's manual, you can try these steps:
1. Identify your engine: Look for an engine code stamped on the engine block (often near the valve cover). This code will help you determine the exact engine.
2. Use an online resource: Websites like the Buick Owners Club or online forums dedicated to classic Buicks may have information on engine codes and oil recommendations.
3. Consult a mechanic specializing in classic cars: A mechanic familiar with older Buicks can help you determine the correct oil type and viscosity based on your engine.
Generally speaking, many older engines like those in a '71 Buick Skylark did well with straight 30-weight oil, or a multi-grade like 20W-40. However, this is just a general guideline and may not be correct for your specific engine. Using the wrong oil can damage your engine. Always prioritize finding the information in your owner's manual or from a reliable source specific to your engine.
