H-R,
Gaseous and I were briefly discussing a similar point 'off-forum' a few weeks ago and I know that this refers to the injected version of the 540 but, for the record, the current version of the RII POH states:
On page 7-9, under Systems Description, Engine Controls (page date 3 Oct 2002) it states:
CAUTION
In-flight leaning with engine mixture control is not allowed. Mixture must be full rich during flight.
So, I guess that Frank must've changed his mind enough for it to go from 'not recommended' to 'not allowed'.
Regards,
B73
PS This is in the 'Systems' section rather than the 'Limitations' section of the POH so I guess you could argue that it's still advice rather than an order but I know what I would (or, more importantly, wouldn't) do in flight...