That looks like “seasoning” you’d get on baking sheets and cast iron. Seasoning is polymerized oils that end up having a high temperature breakdown point, quite a bit higher than the original oil. It’s not an issue, and if you’re willing to put up with it, might actually make it easier to clean since it has non-stick properties.
Otherwise, you want to use a basic (as in acid-base) cleaning agent, ammonia, oven cleaner, etc. should work; bleach might not be a strong enough base. Oven cleaner is made for cleaning this type of thing, but it’s one of those cleaning agents where the precautions are absolutely required, not just a company liability thing because idiots.
In my opinion, if soap and water and scrubby pad don’t remove it, it’s not worth further effort.
I’m pretty sure most of our medications are made overseas.