Originally Posted by Punkin
I'm not sure about regular oil but isn't the trick with synthetic that it doesn't breakdown? Well at least under normal driving and not as quickly. I decided after a few 5k oil changes to just go with the 7k change recommended by Chrysler.
It would be interesting if a purely objective study could/would be published without prejustice. There are so many studies, papers, articles published on the internet and in magazines that contradict each other it is impossible to make a true choice.
I've put 350k miles on a Dakota V6 and 210k on a Ram sport with oil changes at 3k +/- miles. Neither every burned oil and have had no engine issues. Could I have gone to 5k with regular oil? Who knows?
Honestly, it just doesn't matter anymore which oil change interval is "right".
Each engine has different requirements, and even identical engines in identical cars still have different drivers, and different operating circumstances.
Driver A drives his car 100 miles all on the highway at 75 mph, every day, to and from work. His office and his house are just off the highway (turn right twice, you're there). He lives in an area that is neither hot not cold all year round. Can he go more than 3k miles on his oil changes? Hell yeah, but he insists on changing it every 1k miles, because he read on the internet that your car will blow up if its not done every 1k miles, and that's what his grandpa used to do with his 75 Ford Granada, with the inline 6 cylinder engine, and the 2 barrel carb. So yes, he changes his oil once a week. He's happy doing it.
Driver B drives his car 15 miles each way to and from work. The first 4 miles are slow speed driving on surface streets (30 mph and under, plus a school zone), then highway at 35 to 70 mph as traffic goes from crap to clear for 10 miles. The last mile is then surface streets at 35 mph. This person changes their oil between 7500 and 10k miles, and sends an oil sample to Blackstone every oil change to get an idea of how his car is doing, and has been doing so since he bought the car new, and now has 30k miles on the car. The oil samples come back perfect every time. Is he crazy for letting his oil change intervals lapse so long, or is he smart for testing his oil every time he changes the oil? Well, that person is me, so I have to live with it.
Driver C owns a Fancy sports car. Babies the car by keeping the engine rpms low all the time, hoping to get the car to last forever. Changes the oil every 10k miles like the car manufacturer says to. Has it changed at the dealership. Engine has a major internal engine failure, and dies at 20k miles. Dealer replaces engine under warrantee. Driver gets more concerned about the engine, and babies it even more, still having the dealership change the engine oil at the proper interval, still using the same Mobil 1 0W-40 oil, just like they are supposed to. Engine number 2 has the exact same failure as the first engine, this time at 15k miles. Owner goes insane, gets the engine replaced, and sells the car, never wanting another car from that company again. New owner doesn't baby the car, revving it up alot, changes the oil with the same Mobil 1 at the same 10k mile interval, and puts 65k miles on the car without any issues.
The third one is happening to owners of Porsche Boxsters, and is referred to as the IMS (InterMediary Shaft) failure. Basically, with the design of the flat-6 motor, instead of feeding the camshafts directly from the crankshaft in the engine, Porsche decided to create a shaft that goes between the Crankshaft and the Camshafts. One end of the IMS shaft turns the left bank (cylinders 1-3-5), and the other end of the IMS shaft turns the right bank (cylinders 2-4-6).
One end of the shaft just press fits into the engine block, and just turns against a machined surface. The other end has a bearing that is the same construction as the bearings in the pulleys on the front of the engine. It is also a sealed bearing, and contains the same exact grease in it that the pulley bearings do.
The problem is an owner that lugs the engine causes stress on the IMS shaft as the camshafts make their adjustments for better low rpm performance. The stresses on the IMS shaft wears out the bearing, causing its seal to be damaged, and the the lubricating grease for the bearing gets washed away by the engine oil. Then as the miles pile on, the amount of fuel in the oil increases past the 6k mile range from all the lugging, and because they are always running at low rpms, the oil never heats up enough to evaporate it, and the fuel further damages the IMS bearing.
Eventually the bearing fails completely, the IMS shaft seizes, and the pistons hit the valves, destroying the motor completely. A replacement motor from Porsche is between 11k and 15k dollars, not counting labor for install.
So oil change interval or even the type of oil you use isn't nearly as important as how you actually drive the car in relation to when you change the oil.
Just change it whenever you feel most comfortable.
BC.