Originally posted by: exdeath
Originally posted by: Marlin1975
Changing at 3000miles is a waste
Not necessarily. True the average person won't need to strictly adhere to 3,000 miles with new cars and synthetic oils, but it completely depends on the application and environment as is not automatically a waste. Something as simple as an older car with breathers instead of an enclosed PCV system is going to affect oil change interval regardless of quality of oil.
Simply having synthetic oil doesn't mean jack for the filter or the rate at which an engine produces combustion products or ingests dirt.
Great, the oil will last a million miles, but it's carrying a million miles worth of dirt and metal particulate and junk through the engine and through the dirty filter that started bypassing 990,000 miles ago.
Changing by mileage is only an estimate anyway. The real purpose of changing oil, even a theoretical perfect synthetic that never breaks down or needs to be changed, is the suspension of foreign crap IN the oil and the replenishment of detergents and other additives. An engine is not a perfectly enclosed hermetically sealed and isolated environment.
Hence I change my oil based on when it starts to become opaque and darken, which is coincidentally around 3000-5000 miles. That's enough for me to say the oil suspension is saturated and due for replacement. Of course that only applies for an engine that has been kept meticulously clean from the start; you'll go nuts trying to flush an engine out that turns the oil black as soon as you put it in.