lol, they aren't falsely advertising. "Fully synthetic" has been deemed to be a minimum of 70% synthetic compounds. 100% synthetic is just that (which is why Nulon had to change their engine oil packaging).
For reference, I enquired about Anglomoil's "Roadmaster 500" as I have heard good things about it in the past and it is around $55 per 5L tin (yes, a tin, like in days gone by). Here is their response to my questions if you can be bothered reading a wall of text;
The Roadmaster 500 is a Group 3 based synthetic, I don't know of anyone making this type of oil from Group 5 base. Before the advent of Group 3 it was the norm to use a mixture of Group 4 and Group 5. The reason for this is that Group 4 is not a very good solvent for the additives and has limited seal swell characteristics. The Group 5 (Ester) was added at about 10% to enable the Group 4 to perform satisfactorily in these areas. This type of formulation was quite expensive to produce due to the cost of the base oils, the only product that comes to mind made this way is Mobil 1 due to Mobil being the worlds largest producer of Group 4 (Poly Alpha Olefin). The Group 3 base oil is a severely modified mineral oil with the same characteristics as Group 4 and has been determined that it can carry the title synthetic at a more economical price. This approach to most synthetic motor oils in the market is in the majority. There should be no problems with using E85 and Roadmaster 500. You can find a data sheet on our Web site www.anglomoil.com under Products, Petrol Engine Oils.