[Haskell-beginners] fixpoint operator for ULC with specific reduction property

Hello, I'm looking for a fixpoint operator Z for the untyped lambda calculus that has the property Z g --> g (Z g) for any g. The best-known fixpoint operator Y := \f. (\x. f (x x)) (\x. f (x x)) does not have this property, but only Y g --> T <-- g (Y g) for some T. Does anybody know such a Z I'm looking for? Greetings, ben ps. Please keep me in CC, since I'm not currently subscribed to the list.