You could try spray-on, wipe-off oven cleaner. Sold in North America under the name "Easy-Off". I works very well to clean baked oil from crankcases, though some of the older over cleaners could attack aluminum. I think the lemon fresh scent formula works best! Never tried it on charred oil, but if it cleans the burnt Sunday roast from the oven, why not? If it will remove the residual stain too, remains to be seen. But if you have such in the household, it might be worth the time to do a small test.
-Doug