One of the most important things to remember before cooking sirloin steak is to make sure it comes to room temperature before it goes anywhere near the pan. If you cook a steak straight from the fridge, the inside will be cold and take much longer to cook, and it'll likely still be raw by the time the outside of your steak is done. An optimum thickness for a steak is between 3cm and 4cm, any thinner than this proves tricky not to overcook.
In terms of oil, it is best to use a flavorless oil with a high smoking point such as groundnut or vegetable oil. If you want to add the richness with butter, do so after you've flipped the steaks, and baste the steak with the gorgeous foaming butter as it cooks. Try adding herbs such as rosemary or thyme and garlic when you add the butter for an extra flavor dimension.