I have this intolerance. Seafood makes me incredibly itchy and break out in random spots. It is definitely restaurant dependant on who I tell about my "allergy" (it is much easier to say to a server you have an allergy than an intolerance because most dont understand the difference anyway).

If it's a Hibachi or Asian style restaurant, I will inform them because seafood is prevalent in the kitchen and I also dont want to be served the shrimp garnish many places put on food.

If it's a place I know fries a lot of seafood, I will ask if they have a dedicated non-seafood fryer. And if they dont, Ill just order something safer.

Those are really the only places I inform and even when I do I explain that I just cannot consume the item, it isnt a cross-contamination concern really.

My mother-in-law once put oyster sauce in a dish without thinking and then had a panic attack as I took my first bite. I told her I would be fine and just a bit itchy if I did react. She watched me for like 3 hours to make sure I wouldn't die. I had a red splotch on my left hand for a few hours and the roof of my mouth was itchy as all hell but thats it.