There isn't a specific oil or chemical that is commonly used to intentionally shrink clothing or eat away thread in clothes. Shrinking clothes is usually an unintended consequence of improper laundering methods or using hot water and high heat during the drying process.
If you want to shrink specific garments intentionally, you may try washing them in hot water and using high heat in the dryer, but this approach can be risky as it might damage the fabric and affect the overall quality of the clothing.
As for chemicals that eat away threads in clothes, strong acids or corrosive substances like bleach or sulfuric acid can damage fabric and cause threads to break down. However, it's crucial to note that using such chemicals on clothes can be hazardous and is not recommended. These substances can not only ruin your clothes but also pose serious health and safety risks.
If you encounter issues with clothes shrinking or threads breaking, it's best to follow proper care instructions for the garments, including washing and drying at appropriate temperatures, and avoid using harsh chemicals or extreme conditions. Additionally, if you have delicate or valuable clothing items that require special care, it's advisable to consult a professional cleaner or tailor for advice on proper maintenance.