RAM Usage on Macbook
Are these numbers to be expected nowadays?


Every new OS needs more RAM on its own than the older ones. That's because developers are always adding new features, which means they will need more RAM.

So yes, 1.5 GB RAM only for the system is absolutely normal, Windows 10 for example even needs 2 GB. 


  I have always upgraded my Late 2008 Notebook to the most recent OS X version. Could it be that I experience some kind of bloat / fragmentation that is related to this?


I don't think that the updates themselves are causing problems, but simply the newer systems aren't designed to run with only 4GB of RAM, which means that you have to think about always updating to the latest OS.

Of course it will run, but maybe it needs to much resources if you want to run many programs at the same time.