I'd be sooner to suspect environmental contaminants (changing types of air pollution, microplastics, pesticides on food, stress hormones in factory-farmed meat, et al) as a cause.
Although I suspect there's something to your hypothesis as well. I don't know about a spoonful of oil, but I have gone through phases where food seems less "rewarding" on a physiological level, and as a result I feel more satisfied with less.
Air pollution seems unlikely as it's been falling for decades, and America has much lower levels than Europe. Microplastics are a perennial boogey man, and probably a genuine ecological issue, but there has no been mountains of studies done and documented health effects are virtually non-existent. Livestock and slaughterhouse conditions are certainly not good from an animal welfare perspective, but we've had similar practices for 100+ years and obesity wasn't a major issue in 1960. Pesticides in food products have been substantially reduced since the EPA changed guidelines in the 1990s, and yet obesity has exploded since then.
PFAS chemicals are a likely culprit, their presence increased dramatically as obesity rates took off and they are stubbornly persistent in the water and food supply.
Although I suspect there's something to your hypothesis as well. I don't know about a spoonful of oil, but I have gone through phases where food seems less "rewarding" on a physiological level, and as a result I feel more satisfied with less.