There are ASTM performance standards that dictate how the oil
performs when labeled a Synthetic Blend. Back in the '90s Mobile did a spin test on the competition just to see what they were up to. They found Castrol Syntex to contain 0% synthetic oils; 100% "dino" base oils. There was a law suit. Mobile showed their test data and made their claim. Castrol countered with ASTM Performance Standards, and their test data showing their Syntex met or exceeded every ASTM Standard. Castrol used Conoco's Hydracracked base oil, which removed 99.9% of the sulphur. Sulphur mixes with water in the blow-by gasses in the heat of an engine to form sulphuric acid. This of course attacks seals, gaskets, bearings... In short the judge sided with Castrol.
Next day oil companies were changing their chemistries and labels. Mobile1 replaced some of the more expensive full synthetics with the Conoco Hydracracked base oil. Motorcraft's label changed from "Premium Petroleum Engine Oil" to "Synthetic Blend" (as they were already using Conoco's base oils).
There is a performance standards rating on oils. The current standard is SN (devoid of zinc oxide). If it is labeled a "Synthetic Blend", all it means is that it performs up to ASTM standards. I did a
write-up here.
