Put the oil, chilies, garlic, onion, and ginger in a food processor or blender and puree. Pour the mixture into a heavy saucepan and add the sugars, vinegar, tamarind, lime zest, and fish sauce. Bring to a boil, stirring, to dissolve the sugars. Reduce heat and let the mixture simmer until it has reduced and thickened, about 25 minutes, stirring occasionally. Let cool (mixture will thicken more as it cools.) Store in covered jars or containers in the refrigerator for up to 2 months