Lycoming released
Service Instruction 1070R on 16 April 2012 which supercedes SI1070Q and Service Letter L199. SI1070R gives a list at Section B of those engines which are cleared at this time to use UL91. Note that you have to either use an additive in your current oil or swap to Aeroshell 15W50.
If you are operating a CofA aircraft I would recommend you discuss the use of UL91 with your CAMO before you start using it. Always nice to clear the decks beforehand - prevents getting a nasty shock (and a big bill) later on!