Here's a link to an article in EngineBuilder.com that provides some great insights into the difference between the fuels that are sold today vs. those which were the standard back in the day when the original Z's were designed and used...
http://www.enginebuildermag.com/2013/11/impact-of-todays-fuels-n-carbureted-engines/
Among other things, there are some interesting points raised about fuel volatility and ethanol drop-out and the implications these fuel characteristics have for long-term (or even shorter-term) storage of vehicles with vented fuel systems.
The EngineBuilder.com site looks like a great resource for owners who like to pursue performance improvements for their stock engines.