There's a lot more to the issue than viscosity or even "dirt". Modern oils, synthetic or "dyno", contain additives that do things like help keep the engine clean, neutralize acids, prevent corrosion, protect under extreme pressure, etc. The additives eventually break down or are "used up", and changing the oil becomes necessary to replenish the additives. This is the same reason you need to change coolant or brake fluid, even though they are in a sealed system.
The folks who sell synthetics have pushed the idea of extended drain intervals since the beginning, mostly to help convince people to try it and to help justify the much higher price. I've personally used Mobil 1 in all my vehicles for the past 7 years, but I continue to follow normal drain intervals. The only situation where I would consider extended intervals is if I were running a fleet of commercial vehicles that accumulated lots of highway miles over a short period of time.
Jerry
Jerry, I have been running Amsoil in engines since 1978 or 79, can't remember exact date but I was racing dirt bikes and they were supplying me oil then.
I now have a 1987 Ford E 350 van with the 7.3L diesel. It has 517,250 miles on it now. I have run Amsoil 15w40 since 2000 miles. I change the filter every 12,500 miles and then both oil and filter at 25,000 miles. Just had the compression checked before having the glow plugs replaced and the mechanic thought it had a new motor. He wanted to know why I didn't replace glow plugs and injectors when the motor was changed. Had to inform him it was the original motor.
In our old 60 series Detroits, with over 25,000 hours on them we have never changed the oil. It has one of Amsoils external oil filter systems. You loose 1 gallon of oil every time you change filters. I guess the supplements you mention get replenished enough when you top off the oil when changing filters.
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Topic: Amsoil change intervals ??? (Read 4230 times)