Load fryer baskets half full with food. It reduces both undercooked and overcooked food. Don't season or salt foods over the fryer. It breaks down oil. Keep oil at fill level.
Oil is absorbed during each cooking batch.
That's only natural. It's your job to add fresh oil to the manufacturer's recommended fill point and bring it back to temperature. Keep oil at the recommended temperature. Foods fried at proper temperatures taste light, fresh and crisp. Those foods fried at lower temperatures are often greasier and less appealing.