Way back in the dim dark ages, I studied Lubricants , cooling & cutting solutions for a 13 week term a part of a diploma course .
So while I do not consider myself to be expert in any way shape or form I am always amazed at how passionately people whose only knowledge is what is written on the bottle will argue about oils .
Engines are designed for oils with specific characterists at particular temperatures and using an oil that superceeds these specifications is generally a waste of time money & effort and in some cases can cause considerable damage where oil passageways are sized for a particular viscosity to pass a specific volume of oil over a specific amount of time under a specific pressure.
Putting a fuly synthetic oil in such engine can cause a drastic drop in oil pressure because the oil holes and running clearances are too big which causes a pressure drop sufficent to prevent proper boundry lubrication to occur .
Fully synthetic oils are of course are not made from oil that are made from gas .
But what is the gas made out of ?
the exact same bacteria as oil is made from the only difference is the time of the deposition & the pressure the deposit has been under .
So if you are making a jet engine or an F1 engine where running clearences are down to hundreths of a thou, they make a massive difference because standard oil is too thick to get between the metal surfaces
However mower engines have clearances in full thous so definately not needed
In a hydro box where the oil has to pass between the flow holes at a greater rae than between the cylinder & valve chest then the tighter specifications of fully synthetics is important .
Even then many of the fully synthetics are actually made from waste refinery gas so they started life as an oil, got gassified then returned to an oil again.
Engine oils are the most complicated liquids in common use second only to printers ink.
Any ameteur ( mself included ) who thiks they know it all about oil is playing with parts of their anatomy that mummy told them would send them blind if they did not leave it alone.
So if any one wants to put a fully synthetic, semi-synthetic oil into their engines and it makes them feel great doing it then please go ahead an do it .'
Happy people are a lot more fun to live around but please don't delude yourselve s that it will make a tinkers cures of a difference to the life or performance of your mower engine .
Mower engines are the cheapest most basic underpowered engine that it is possible to make.
ZDDP is not a particular problem to catalytic converters in the volumes used in engine oils .
The volume that sticks onto the cylinder wall then is passed into the exhaust and expelled by the engine is so miniscule you could measure it in individual molocules and has been found to have little effect on the converter
The problem is EPA regulations that consider it a "Heavy Metal Pollutant which may cause cancer in the state of California"
And yes it has been found deposited on the surface of the cat but in such small volumes that it makes no effective effect on the operation of the cat.
However under lab conditions where 100+ times the volumes that could ever be deposited by an mower engine running 24/7 for 100 years was passed over a cat it did diminish it's efficiency