Using a thinner oil like 5W-20 can lead to several problems:
* Increased oil consumption: The thinner oil may not provide sufficient lubrication at higher temperatures and pressures, leading to increased wear and oil consumption.
* Increased engine wear: The thinner film of oil may not adequately protect engine components, resulting in increased wear and tear on vital parts like bearings and camshafts.
* Oil leaks: The thinner oil may be more likely to leak past seals and gaskets that were designed for a thicker oil.
* Reduced oil pressure: The lower viscosity can lead to reduced oil pressure, particularly when the engine is hot, further increasing wear.
Always check your owner's manual. It will specify the recommended oil viscosity for your specific 1987 Thunderbird engine. This is the most reliable source of information. If you don't have the manual, you can often find a digital version online.
In short, using 5W-20 in an older car like a 1987 Thunderbird is a gamble that's very likely to result in engine damage. Stick to the recommended oil viscosity.
