I have always used 5 w 30 motor oil during the winter and summer months. My engine is old [200,000miles] but runs very well and starts very easy. my book says i can use 15 w40. is this oil better or would it move too slow during winter. It is not too cold where i live 40 degrees but i am going to visit in the north. 25 degrees. Does any one use it? should i use high mileage motor oil? I also like using Royal Purple.