If the oil level was high, and you're sure it wasn't overfilled, then it had to be gas contaminated. It got in through a bad fuel pump diaphragm or an extremely rich carb, (choke not opening, flooding). A mixture that rich would have been running bad, black smoke, for along time.
Gas contaminated oil is plenty good reason for the bearings to fail. The oil would have turned black from extreme overheating during the failure.
Synthetic oil is just fine for those engines, even better in my opinion, it resists heat better, but no oil can withstand the kind of heat a bearing failure generates.
As far as idling down the engine before shutting off, I have always done that, and will continue to do it! Never caused any problems. Would anyone rev their car/truck engine to 3600 RPM before shutting it off? LOL