Synthetic oil bad in a lawnmower? Any truth in this self proclaimed experts statements?

Damm, what a load of BS. I have two Honda small engines one V-twin 20HP and one single 8HP, along with a generic lawnmower and a Yamaha golf cart. All these engines have used 15-50 M1, 15-40 diesel oil, M1 10-40HM and now M1 10-30. Some are 20 years old and never an oil related problem. All are small engines with no oil pump or filter except the 20HP Honda. They are happy with most any type of oil as long as it is clean and changed reguarly. Probably going to use 10-40 Supertec syn next, depending on availability.