Many buyers assume thicker paperboard prevents grease stains, but grease migration depends more on barrier performance than GSM. This article explains why oil can still penetrate 400gsm paperboard, why packaging samples may pass while retail packaging fails, and how transportation conditions affect grease performance. Through real packaging examples and industry testing principles, it shows the difference between structural strength and grease resistance. Buyers can use these insights to avoid unnecessary material upgrades, reduce packaging complaints, and make more informed food packaging decisions.