I have a 98 LS400 I brought used with only 18,000 miles. I started using synthetic oil last November. I think I am using 10w30 Mobil 1 oil. I live in New York, I think the oil should be fine? How long can I drive before change the oil? Right now, I use the 5000 miles 6 months interval. I put on about 5000 to 6000 miles per year.