The vehicle is a 1991 toyota previa. I changed the oil about 5 months ago with chevron 5w30 dino, but the oil only has about 1200 miles over those 5 months (the van doesn't see much use lately). It hold about 6 quarts of oil, btw. Keep it in there with consideration of milage or time? I'm concerned with oxidation, etc, from it sitting too long. I do get the oil up to temp about every other week by running on the freeway for 15 minutes or so. Thanks in advance.