Oil preasure light goes on after oil change and stayes on until the engine is warm

Hello. I changed the oil in my golf 3, 1.4L, 1997, and 2 weeks passed since. After i changed the oil, the oil light got on and it stayed like that until the engine got warm. It does the same thing every time i start the car. I took the car to a mechanic, and he sayed that he doesn't know why that i shappening. I saw a video on youtube about dirty oil can couse that, but i don t know if is true, especialy after i saw the guy cleaning the oil bay putting kerosen in the engine. Please help!