I'm sorry Cab, its the other way around.
Here's an article from a former Shell fuels guy about some of the testing done real motors as well as the standard lab tests.
https://generalaviationnews.com/2015/03/24/testing-the-octane-rating-of-aviation-fuel/And a general article on Avgas characteristics with links to references.
https://www.experimentalaircraft.info/homebuilt-aircraft/aviation-fuel.phpThey claim all traditional Avgas has (had) a density around .719 g/L at 15*C (59*F).
Its interesting that BP's pamphlet on UL91 states its Motor Octane number is 91 and that it has a research octane number of 96. That would be an anti-knock index of 93.5
It seems odd that the rich mixture rating isn't given, but maybe thats not a requirement for aircraft that have an STC for 91UL.
Unleaded UL91 pdf 
