So is it the spices or the bacterial environment?
Sven, do an experiment for us, please. Eat the blandest food you and get hold of for a week and get back to us on the results.
Diarrhea is multi-factorial.
More data here: Acute Diarrhea
For chronic diarrhea, the causes are generally broken down into 3 groups:
Chronic Fatty Diarrhea
Chronic Inflammatory Diarrhea
Chronic Watery Diarrhea
Each of those has its own particular etiologies, causes, and treatments.
Chinese food in general is fairly hygienic. Raw vegetables are only served if they are pickled, and fresh fruit (including grapes and sometimes even strawberries!) is always washed, dried and peeled. Cooked meat and vegetables are cooked through using extremely high heat. Drinking water is served hot from being freshly boiled. Street food is nearly all hot and freshly cooked in front of you. It is rare to touch your food, and food requiring fingers is often served with plastic gloves. China’s indigenous sanitation systems are pretty thorough and food poisoning is relatively rare.
It may be the oil, it may be the spices, it might even be the crappy beer, but it’s probably not (usually) bugs.
I can always rely on you to remind me why I didn’t go to med school. 
So… oils, spices, or bacteria?
Seriously though, bacteria doesn’t explain eating spicy food in 1st world countries for anyone, and even in 3rd world for natives - when I visited Honduras they didn’t seem at all concerned about the local water for their own consumption, but they made sure that I didn’t drink any - clearly they have gotten used to the local bugs.