Looks festive, right? And it was...until the next morning.
Our Puppy Dog appeared to be fairing the best of the four (cooking oil). Frankenstein's Son (no treatment) had a small colony of flies beginning to take residence, Kitty Cat (petroleum jelly) had a dry funky mold at the top and Harry Potter (bleach/water) developed a pink mold unlike the three others. But this was just the beginning of the end.
We would have loved for the experiment to have lasted longer and seen more rot, but the flies coming of Frankenstein's Son made documenting their decay harder (we may have unintentionally eaten a couple - eww). Ultimately we think the Puppy Dog faired the best out of the four, but we wouldn't say Cooking Oil protected the pumpkin much more than the Petroleum Jelly did. It's our opinion that the untreated & bleach/water treated pumpkins did the worst, suffering from the most mold and flies. Although we had no strong winner, there may be something to be said for covering a pumpkin in an oil like Cooking Oil or Petroleum Jelly, at least to help keep the pumpkin from becoming a comfortable host for bugs.
Our recommendation for Fall 2019? Let's try it again! Maybe there will be less rain or cooler temps. Maybe we'll keep them inside next time, to see if the flies will stay away. We can say based on this year's observation is that an oil-based preservation method won't prolong a pumpkin by much time, but it can help a bit to keep your Jack-O-Lantern a bit nicer for a bit longer.
Can your Jack-O-Lantern Last Longer? Part 1
Update: Can your Jack-O-Lantern Last Longer? Part 2