Diesel oils zinc & phos levels have come way down also. Any motor oil SN has only 800 ppm of Zinc & phos inwhich is very low for any marine app. That is correct about ZDDP will kill at catalyst over time hence for the reduce levels.

Oil groups - group III is really not a syn oil, its more like a hydro cracked oil, Group IV oil based is a PAO and group V is ester based. Driven oil (ex joe Gibbs oil) is a MPAO inwhich is claimed to be a very good oil. Lake Speed Jr. heads up that program.

Mercruiser new recommended 20w -40 syn oil for their catalyst motors also has reduce levels of ZDDP compared to their orignal 25 w 40. In the marine world there is alot of things that will ruin the catalyst.

Run the boat out of gas and you will burn a hole straight thru the catalyst sub strake also certain gasket adhesives will hurt them, also backfires can burn a hole thru them, worn spark plugs over time will the hurt the catalyst and any water reversion will kill the cat on the spot among other things. FYI.