Arrived late but staff just checked me in. Work done in reasonable time. However, I do have a question. As I drive about 4,000 miles/year I wonder how often I should do an oil change if synthetic oil is used. I asked around the time it was due but not even close to mileage, someone had Lex Brodie's told me to go by mileage and not to worry. When I asked yesterday as it has been over a year since the oil change was done, one of the workers suggested to use time as the rule of thumb. So my question is, which is it and why? If it is time, can it be longer than six months?