Are any year Toyota or Lexus model parts interchangeable with a 1999 Lexus RX300?

Yes. Example. The mass air flow sensor on your Lexus says Toyota right on it. If you needed to replace it you can get it through the Toyota parts dept and save some money. The mass air flow sensor from Lexus IS going to be the exact same part that the Toyota guy handed to you but you will pay more for it.