As I understand it, your typical semi-syn oil from a large company is about 10% syn/90% dino. I fact they could add a couple of drops of a group 3 oil to dino and call it a semi-syn oil. They do not list the amount on the bottle for a reason!
Yes, which means the percentage of dino could be higher than 90%. However, some blenders put as much as 25% syn in their brew. It's probable that the "syn" in these blends IS Group-III, not PAOs. If you want to run a blend of known quality, run a bottle of M1 with the rest DriveClean. If your sump holds five quarts, you'd have a 20% blend of PAO-based synthetic (whose carrier is also synthetic) and the additive packages should be as compatible as possible for a brew-your-own.
That's why the only semi synthetic to consider is Schaeffer Supreme, since we know it's 70-75% group 2+ and 20-25% PAO. Their 5w30 blend could actually be labelled fully synthetic if they wanted to, since it's 70-75% group 3 and 20-25% PAO. But Schaeffer Oil doesn't want to decieve people like other oil companies do.
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.