Let me first say how much I enjoy this form and how amazed I am at the knowledge of its members. For my first question. I would like your opinion on how long I can go between oil changes.. I have a 1996 Topkick with 180,000 miles and a 215HP cat 3116 engine. I drive about 3000 mi per month and use Shell Rotella 15W40. The recomened interval is 6000 mi, which seems very short for a motor that has a capacity of 20 liters of oil. I have done 2 oil analysis-- 1 at 6000 mi. and 1 at 8000mi. I have posted the results below and would like some advice on the results and the oil change interval.