Unless the bottle is labelled or the manufacturer posts a percentage, I wouldn't assume anything.
IMO, any company willingly printing percentage amounts is worth buying from, if you want to use a blend, and save money over the socalled full synths.
Its depressing to think that you could have .5% synth oil mixed with mineral, and call an oil BLENDED.
If I wanted to use a blend, I'd blend my own. Oil soup hasn't hurt any engines that I maintain.