Is an Oracle SID UPPERCASE or lowercase? This may sound silly, but is an Oracle Service Identifier (SID) aka database name normally in upper or lowercase? 

According to the documentation, a SID "can contain only the characters a-z, A-Z, and 0-9", so both upper and lowercase is allowed. In my experience, it usually doesn't really matter. However, I am installing Oracle 11.2 on a file system that is case sensitive, and a lot of directories and files have the SID in them (e.g. initMYDB.ora). 

EDIT:

I found a quote saying that the SID in the password file name (orapwMYDB) is case sensitive and must match the instance name.
Case sensitive on Linux/UNIX and not case sensitive on Windows.