I've got a fixed wing PPL and did a trial flight in a heli, but the instructor told me I would have to log it in a separate rotary log book, not the same one.
I also did a session in a full A320 sim and was looking forward to having a big jet in my log book (
) but the guy who did it, although an airline pilot, said I couldn't put it as PUT as he was not appropriately qualified.
However, it is your log book, so you can pretty much put what you want in it - it only becomes an issue if you try to claim for hours that you're not entitled to - I think!