What type of oil do you use? In the winter, with my other cars I liked to run Mobil 1 syntek 10W30. I have heard that there may be an extra measure of protection with it because it keeps a thin film on cylender walls, etc. at start-up. Also I heard about better mileage, and it will not freeze in extreme temps. Is it true that when you use synthetic oil in a higher-mileage car that it is thinner, and may leak from valve covers, etc.? I think that some new cars like Corvettes and Mercedes Benz come with synthetic off the showroom floor? Do you think it is worth the extra cost?