This and most of the rest of the discussion is off the mark. If you change oil at 3000 miles it is because you are interested in what the engine will be like in 300000 miles not what the oil will be like in 5000 miles. Most people don't care what the inside of the engine looks like in 300000 miles - why should they? If you are a typical new car owner you sell the car long before you discover the consequences of the maintenance intervals you use. A
7000 mile oil change interval is adequate for most new car owners. To change any more than that does them no good and will likely only benefit the 3rd owner of the vehicle.As far as I know the type of dirt that gets into oil due to combustion byproducts is not going to be any different for synthetic oil. If you use cheap oil and filter and change every 3000 miles after 300000 miles the engine will be as clean or cleaner on the inside than the day it was driven off the lot. That is because there will not be any accumulation of grime over time. From what I have seen about 90%-98% of vehicles with more than 120K miles are not so clean on the inside. Wipe your finger on the inside of these engines and you no longer have a clean finger. I don't need to know what those cars have had for maintenance, but I'm pretty sure it definitely isn't 3000 mile oil and filter changes.
-jim
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