I know in cars the oil is graded with and SAE service classification, check your present oil for one listed on the bottle, or in your manual and see if there is one one the bottle of the marine stuff. it goes in alphabetical order so for instance a rating of bc would be higher than aa Higher is better.
The only difference between synthetic and petroleum based is the shape of the molecules and the way it is made, petroleum based allthough slippery, on the molecular level has square and oblong shaped particles(picture marbles and building blocks mixed together).
A synthetic on the molecular level has a consistent marble shaped particulate. obviously better, like a giant, liquid bearing.
I am a fan of full synthetics, I have researched them and discussed them with engineers from auto companies who interact daily with oil reps and technicians. they say synthetic is excellent except when breaking a motor in, then they say it is too good, it wont let the rings seat.
Just my .02
Tim