The 91/96 UL aviation fuel available in Europe is already approved for use in all Lycoming engines originally certified to use the old leaded 91/96 avgas, as well as all 80/87 octane engines. See Lycoming
Service Instruction 1070N. Only engines originally certified to use no less than 100/130 or 100LL are not approved to use the new 91/96 UL aviation fuel. Further, given Sunoco's success in producing an unleaded 100 octane racing fuel for NASCAR, I should think that a 100 UL avgas is achievable with current technology. I for one will be happy not to have to pick the lead globs out of my spark plugs every 50 hours.
Your service letter states that UL will not harm the engine,
it states
quote
The importance of using the fuel specified for a specific model Lycoming engine has always been stressed
in Lycoming service publications. However, if the specified fuel is not available,
a higher grade fuel may be
used, subject in some instances to the restrictions described in the footnotes to the following Table of
Specified Fuels. The chart showing specified and alternate fuels that can be safely used in no instance
permits use of fuels of lower grade than that which is specified. Also, it is not permissible in any instance to
use automotive fuel in aircraft engines, regardless of octane or advertised features because of the corrosive
effect of its chlorine content and because of vapor lock that could result due to its high vapor pressure. Any
fuel used in Lycoming engines must conform with Specifications ASTM-D910 or MIL-G-5572F.
But will you also be as happy to loose the preformance? the SB doesn't say it won't cause lower horse power.
Remember the TSIO 360- Cont was already de-rated to use 100LL
Simple BTU theory tells us that UL does not have as many BTUs per pound Pure Gas stock. So you will be required to burn more of it to do the same job, which simple logic tells us, the legs on your aircraft will be shorter.
And secondly, We don't have 91/96 UL here now, nor will we in the short term.