Nope, don't mind at all.
Sunbae is used if you're addressing someone in a more formal term -- for example someone who is a grade or two ahead of you in school. Or someone who came into showbiz earlier than you did. This is why Joong Ki was calling Shin Hye "sunbae" initially, because although she's younger she started earlier than him in showbiz. So, that's the context of Sunbae - it's more grade/level related. Oppa is used for age-related.
Does this help clarify things?
An acquainted guy you're not too close with -- you can use the formal term I think -- adding a -ssi suffix after their name. For example, Shin Hye might call Hyung Joong-ssi by this if they are not close.