So I had purchased this truck last July, and within a week the oil change due indicator came on. I had it changed with conventional oil but since have been doing my own changes with full synthetic. The first synthetic oil change due indicator came on within 3000 miles. The last two i've done had no indicator warnings at 3000. My question is; does the system have a sensor to let you know when oil change is due? Is it based on a sensor for oil viscosity or does it just spit it out based on mileage? How long should the full synthetic last? Thanks!