After several tests, it really seems like oatmeal is the best thickener.
Because if you use any other starch, corn/potato/yam, after 1 or 2 days in the fridge, the starch bonds are quite loose, and the curry becomes watery, even if you added lots of starch to it initially.
WHEREAS, with the oatmeal, it seems after a whole week, the bottom of the bowl is still gooey and very cling-on for the rice. And it retains that mrrrphhrrhrhrr feel in the mouth.