Can I use regular unleaded (87 octane) gas for BMW 525 i (2003)?

Yeah you can use it with no problem. I have a 2002 325i and I have put 87 octane in it since I bought the car three years ago and have had no problems. The engines are designed to compensate for the lower octane. I talked to my mechanic, who only works on BMWs about it a few years ago to make sure I wouldn't wreck the engine and he said that it will have no ill effects on the car other than a slight drop in performance that you probably won't notice unless you are an extreme driver. He said he even uses 87 octane in his BMW since it doesn't make a huge difference. Go ahead and use it and save that money for the wonderful repairs that come along with owning a BMW.