Here's the skinny on synthetics;
Mobil 1 used to be a true synthetic base stock oil. Meaning that it used a synthetic, lab created base, plus it's additives.
Mobil 1, Castrol Syntec and Pennzoil synthetic are not a synthetic base. This means that they are not a true, full synthetic oil and have not been this way for well over a decade, now.
The laws have changed in the United States as to what type of oil can be classified as a synthetic. This means that a crude oil base stock can be classified as a synthetic oil, because it is refined with synthetic bases as one of the additives, rather than a majority base stock to start with.
There are three types of actual synthetic bases that real synthetic oils are made from.
Everything from API Group 1, 2 and 3 are crude base stocks. Group 3 base stocks are treated with very high molecular refinement and have close properties to Group 4, but are just shy of it.
Group 4 is where true synthetic oil begins. The types of synthetics used today as a base stock are Polyalphaolefin (PAO) and/ or Ester.
Mobil 1 is a crude base stock oil, that falls under group 3 refining. The US courts have sided with petromechanical companies, classifying crude base stocks that are refined to API group 3 standards as a "synthetic" oil, when in reality, they are, without a doubt, crude base stock.
The relevance of synthetic oil in an engine has to do with what components the oil is being used with.
Most synthetic oils, and a lot of crude base stock oils that are widely/ easily available are void of the additives in the additive packs used in them, that old engines need.
Zinc is one of the additives being omitted by the majority of manufacturers, as well as equally, if not more importantly included phosphates being left out.
This is because new cars have catalytic converters that these heavy mineral additives can bond to the platinum in the element and inhibit a catalytic converter's ability to remove hydrocarbons from the gasses that it contacts.
So really, what you should ask yourself, is what is in your oil, regardless of synthetic or crude base stock, against what is in your engine.
Flat tappet and looser bearing designs, made from softer metals need the minerals in the additive pack to keep the engine lubricated as it was designed to be.
New engines have everything rollerized, as well as fuel injected, have tighter bearing tolerances made from aluminum and less forgiving materials, so you don't need the additives in those engines, because they are more controlled.
Newer oils inherently have more detergents in them, in percentage, so the tighter tolerances and less forgiving materials can live cleaner. This is bad news for mineral additives, because the added amount of detergents pull them from the surfaces, along with the carbon they're designed to clean.
My advice to you and anyone is to get an oil report from a lab on the type of oil you have your preference for, to see if it contains the correct amount of each additive, for your engine.
The truth is, unless you know what is in the oil, currently, you actually have no idea what you are doing to the engine.
I destroyed a stock cam, using Mobil 1 with ZDDP additive, in a flat tappet 318. I'm convinced that this was because of the high detergent levels counteracting the additives.
Do your homework or cross your fingers.
