The oil pressure sender on my 240z has already gone bad, so I'm replacing it soon, but I'm afraid it might break off inside the block when I attempt to loosen it. What's the best way to remove it? Should I do it when the block is hot or cold? And is there any type of sealant I should use when installing the new one?

There is a hex on them just above the threads. I forget the size but an open end wrench will remove it. No sealant.

You could spray it with PB Blaster the day before removal.