Sounds like it was building heat, and leading to pre-ignition. The piston was so hot the fuel mix was vaporizing in the crank case. You must know your way around engines some to recognise a problem developing. The next sound you would have heard was a piston seizing. Poulan to my knowledge uses Opti2 and remarkets it. Opti2 is one of the first if not the first to design synthetic oils. Still in business after 30 years so they probably know what they are doing by now.
Solutions would be, Smaller prop, try some fresh gas from a different source, back the timing off a couple degrees, tune the high a bit ritcher. It's summer and you should expect quite a bit of performance loss with air density being thin.