Advertising transparency, a key factor influencing the effectiveness of YouTube influencer advertising content, is largely affected by ad disclosures. However, despite the diversity of disclosure types and levels of transparency, existing literature primarily focused on examining the effects of simple dichotomous differences—such as the presence versus absence or low versus high levels of disclosure. The results of these studies have been inconsistent and often controversial, which may be partly due to the differing operationalizations of disclosure levels across studies. In this context, this study classified ad disclosure levels into three categories as low, medium, and high, and examined how consumers’ actual responses to influencer advertising content varied across these levels. The findings reveal that content adopting YouTube’s standard ad disclosure format (medium level) generates more positive consumer responses across multiple advertising-effect indicators. This is noteworthy, as it refines previous findings that merely compared low and high disclosure levels, suggesting that there may be an optimal level of disclosure that positively contributes to advertising effectiveness. In other words, for long-term advertising effectiveness, an appropriate level of ad disclosure—rather than excessive or insufficient disclosure—appears to be most desirable. Future research should further explore a wider range of disclosure types and levels to deepen understanding of these dynamics.