Please tell me if you can, I am thinking about changing oil in my new car. Which now has 8000 miles on it. The oil and filter was changed at 1000 miles, then again at 3000 miles, so on and so on.and now it has 8000 miles on it. So, I believe the engine is broke in now. For years, in my other vehicles I have used Castro oil, usually 10-30. My new car is a Mazda miata. It is a four-cylinder engine with twin overhead cam's. It has a red line of 7000 rpm!I do drive the car hard at times, usually shifting at around 6500 rpm. So, I would like to run some type of oil that would keep my engine in good shape for years to come. Should I use a synthetic oil? Possibly Mobile one 10-30? The car owner manual says to use 10-30 weight oil. The only reason I mention mobile one, is it is locally available [Wal-Mart and auto parts stores locally] in my area of Texas. Do you think that mobile one is a better oil then Castro oil of the same weight? Can you point me to a WebSite, something like Consumer Reports or some other WebSite with unbiased opinion? Thanks again. PS, some people say you can use mobile one and change it and filter at 5000 miles is this true?