According to the system requirements, even Java 7 requires Lion, but then I found this question: How to install java jdk 7 on Snow Leopard. Especially the answer stating that java 7 doesn't, in fact, require 10.7 and that this restriction can just be removed from the .pkg gives hope that it is possible to install even Java 8 on Snow Leopard. Any information on this?
The OS level requirements are "real" and shouldn't normally be circumvented. As to how real it is I cannot answer. Ultimately as system libraries are updated, and newer versions of JDK are compiled against them older systems won't be able to use parts of Java that use the newer functions in those libraries or even the libraries as a whole if the pointers/links have changed.
If you want to be absolutely sure Java will work on your system, you will need to compile it from source. But that is a massive pain on a Mac unfortunately. If you want to try anyway, check out the Mac Port Project over on OpenJDK.