You should feed your dog sardines that are packed in water. These are the healthiest options. Sardines however are usually packed in oil at the supermarket. Generally dogs do not have a problem with oil but you might want to avoid giving them too much fat.
Dogs should consume 10-15% fat in their diet as a general rule.
The health benefits of olive oil outweigh those of sunflower oil. The antioxidants in olive oil for example are much higher than those in vegetable oil. In addition olive oil contains monounsaturated fats which are anti-inflammatory and promote heart health. Sunflower oil is high in polyunsaturated fats which can lead to heart disease and inflammation.
You can remove the sardines from the can and put them on a paper towel to absorb the excess oil if you are concerned that your dog is getting too much fat in his diet.
When you find sardines in water be sure to read the label and check the salt content. Make sure the product is free of salt flavorings and preservatives. Thats why you should not feed your dog sardines in brine they contain too much salt.
Read the label carefully on sardines in tomato sauce since some of the ingredients might not be healthy for your dog. Commercially available sauces should generally not be given to pets. Sugar salt and chemicals are usually common ingredients.