I'd say its a mixed bag of watchers, which I guess means nobody at all. How effective it is, is anyone's guess. First of all, if advertised claims are too far away from reality, oil companies have shown a lot of willingness to take each other to court. Pennzoil had a Brett Farve ad campaign that Castrol took to court, as part of a wide ranging suit that insisted that Pennzoil was out of line (I don't recall the exact charges). Valvoline has also taken on Prestone (either in US courts or in these quasi-judicial tribunals that our government sets up like the FTC) for false claims about whether P.'s coolant is actually suitable for all vehicles.
Then there is teh PQIA, which of course takes money from oil companies, but takes money from many of them. They test everyone's oil (as actually sold in bottles in American stores) and publish the results. To a certain extent, this keeps everyone honest. They mainly nail the small motor oil companies that many outside of BITOG have never heard of. The only big company that I recall had a horrendous result was Valvoline's nextgen that had one spec that was totally out of whack. Now this will not help you with synthetic vs. blend vs. conventional, but they do, for instance, publish NOACK volatility. If that spec is well below 15% (which conventionals have to hit to be SN), you can bet it is synthetic that is making the difference. Most conventioanls come in around 14.5%.
At this point, I only trust what is on the bottle. If it means a lot to you to have a synthetic blend that has a lot of synthetic, I would go with Pennzoil Gold, which openly on the back states that the base oil is 50% synthetic. Because it is actually stated, and not simply given me over the phone by a customer support person, I believe them.
I'm not aware of any number that a synthetic blend "has" to be. I've heard a few say there isn't any given percentage, that it could be a drop of synthetic; I've read others here give very low percentages. If you are ok with less than the 50% that you could get with Pennzoil Gold, you could use Dexos certification, which requires a 13% NOACK volatility, which to my knowledge has not been hit by any conventional oil. So there woudl have to be a decent amount of synthetic that would allow any oil to meet that spec, to gain DEXOS certification.
