I know that there are many variable such as engine as well as oil but in general can most any conventional oil go 5000 miles? I drive about 25,000 mixed miles a year and was wondering if changing at 3-4k is a waste? It's 3.0 ohv ford that is a service car for me. I don't want drain good oil if I don't need to and know an oil analysis would tell everything but just wondering if anyone had insight on this? I just got it and has 110k on it so I'm a little worried about switching to semi/full syn and developing leaks.