Salt should generally be added *after* frying. Adding salt to hot oil can degrade the oil and potentially affect the frying process.
Adding salt directly to hot oil can have several negative effects. First, salt can cause the oil to break down more quickly, reducing its lifespan and potentially affecting the flavor of subsequent batches of food. Second, salt can sometimes cause splattering as it reacts with the oil. Finally, it's difficult to evenly distribute salt in hot oil, leading to uneven seasoning.
The best practice is to salt your food immediately after it comes out of the fryer, while it's still hot. This allows the salt to adhere better to the surface of the food. The residual oil on the food helps the salt stick, and the heat enhances the flavor absorption. You can also pre-salt the food before frying, but be mindful of the potential for moisture to be drawn out, which can affect the crispness.
Use a fine-grain salt for post-frying seasoning. Fine salt dissolves more quickly and evenly distributes across the surface of the food, providing a more consistent flavor. Avoid coarse salt, which may not adhere as well and can result in pockets of intense saltiness.