Thought I would bump this old thread up. Synthetic is still the way to go, even if you have an older engine. Depending on which brand you buy will vary on the mileage. Since this old post Amsoil has now raised their mileage to 10,000 miles. The even make a longer one, but 10,000 is plenty for me. But you also need to get an oil filter that can last as long as well. You can't expect to buy synthetic and just use a standard 3,000 mile oil filter. Read the box, make sure that the filter is engineered and has the proper filtration materials to handle the longer life oil. But once you use synthetic you won't go back to a standard oil. To many benefits. The downside is you are paying more for the oil and filters.