Thanks for the reply. I hadn't really thought that the dipstick would go into the bottom of the oil pan, but it makes sense.
I'm pretty sure that the heater is filled with antifreeze. The lower hose connection definitely had antifreeze in it, but the upper hose connection didn't really have anything in it so it was hard to tell.
I wasn't planning on doing the impeller. You do yours every year? It doesn't seem very easy to get to.