Beware of what the US calls synthetic oil. Group III synthetics are not really synthetic. That's why you can use them with alcohol based fuels, where a true synthetic doesn't like any alcohol based fuel.
In fact, there are relatively inexpensive group IV synthetics that work ok by themselves but a quality synthetic is a blend of different synthetic base stocks.
Then, as an engine oil, it has to have certain additives, viscosity index improvers, detergents, dispersants and such. All those components come in every cost and quality, just like the base stocks. That's why cheap oils are cheap. They use the lowest quality base stocks, additives VI improvers and such. And, they use the absolute minimum of these additives to make the UPI grade. It all saves costs but a cheap oil is just that.
Does it mean it doesn't work? Even cheap oils do enough to keep stuff from metallic intercourse. It's the other things like valve spring life that shows up with better oils.
