What is an indicator of stale gasoline?

A varnish-like smell is a key indicator of stale gasoline. Over time, gasoline can deteriorate due to the evaporation of lighter hydrocarbons and the oxidation of heavier components. This process can lead to the formation of gums and varnishes in the fuel, which creates a distinct varnish-like odor. This smell is a sign that the gasoline has degraded and may no longer perform efficiently in an engine.

In contrast, while a transparent appearance might suggest that the gasoline is still liquid and not contaminated, it doesn't necessarily provide insight into the age or quality of the fuel. A strong chemical odor can occur in both fresh and stale gasoline and does not specifically signify degradation. A dark color can sometimes indicate impurities or contamination but is not a definitive marker of stale gasoline, as some fresh gasoline can also vary in color due to the formulation and additives used.