The ones to get are the Tickford AU Falcon engines. They are very, very durable.
I am no expert, but it seems by AUII and definitely AUIII, every XR8 had a hand built Tickford engine in it.
The Tickford engines were individually hand built, blue printed, spec'd with roller rockers, GT40P heads,
oil cooler etc.
You can spot a Tickford engine by the Tickford logo on the throttle body. Also there is a Hand Built plaque on the rocker cover with the name of the person who built the engine. If it still has the plastic engine cover, it has a large Tickford logo on it as well.