Eclipse uses a built-in compiler rather than javac. The built-in compiler obviously doesn’t support Java 8 yet.
You can define a custom builder for your project which will call your javac. In the project’s preferences, go to “Builders”, add a new custom builder (which can be either an Ant script or an executable program) and disable the default built-in Java builder.
Afterwards, running the project with the JDK of your choice shouldn’t be a problem – adding it to the “Installed JREs” preference page (as you did) should be sufficient.