I wish I had advice, but can only offer something similar from when I was a
tech. The PI said to me that she expected me to work late and do weekends when
things were really busy, and that when things calmed down I could take the
overtime off as unofficial comp time.
Now, one of the things I really appreciate about academia is the ability to set
your own hours, and to work deals out between yourself and your boss ("Can I
work the Monday holiday and take the next Friday off?"). If what she had said
were true, I would have been fine with it, however, there was _never_ a time
when things 'calmed down'. I finally started working (carefully) the 35 hours
a week I was paid for, and she decided I was slacking.
I think we had a communication problem.
*sigh* He can change your status, or you can work the legal 40. If you do
anything different, you'll resent it.