Many commercial outfits only change oil when a lab test of the oil indicates a change is needed. The time and mileage life therefore depend on the mechanical quality of the particular engine, the chemical composition of the particular oil being used (additives etc) as well as how properly it is driven.

Consequently there is no strict answer to your question and a time life cannot be arbitrarily assigned.

If you want to replace your car every three or four years, then go ahead and listen to anyone that has an opinion. Change your oil as infrequently as you dare. Your going to get rid of the car anyway, right?

If you want your car to last 150,000 miles, or 200,000 miles, well heck X00,000 miles then the three months/3000 miles routine is for you. Yes you will drain oil out that won't look all used up, and that's the point. Replace the oil BEFORE it cannot do all of the jobs that it is supposed to do and your engine will outlast the rest of the car.

Of course there is always a possibility that something mechanical can suffer a failure. I'll be the first to say, transmission services every thirty thousand miles will not prevent you from needing a transmisison repair/overhaul. but if a repair would be necessary, it is much easier, and less expensive than one that is fully negleted. Engines are the same way, if it is completely and correctly maintianed, and it breaks it is easier to repair. If it's neglected and it breaks, well it usually cannot be simply repaired, and it ends up needing replaced. It also goes to say, that if you ignore or are furgal, or lazy about oil change services, then you probably ignore others too so when the engine breaks, you really do need a new car instead of just fixing the one you have.

john whats your opinion of the new g.m. cars that have the message that comes on on the dash and tells you when they need an oil change. my chevy is 9 months old with 3200 miles on it and the dash message says i still have 48% oil life left. dealer said change it when the dash tells me to. just curious as to you thoughts? if it were a car i bought i would still do it 3 to 5000 miles, but since this ones a lease, i dont really care about longevity since i`ll turn it in in another few months.

To the original poster, how is this vehicle being driven? If the car is being driven such short distances that it never gets up to operating temperature, sludge may form whether you change oil often or not. More often would give you a better chance though. You might consider staying with your once a year oil change using synthetic oil. Don't think I've ever heard of sludge forming using synthitic. For most people I think it's a waste of money but it might be appropriate for you.

john g, I agree with you 100% for anything close to a typical driver, however, @ 4000 miles per year, it will take this person 50 years to get to 200k miles. I doubt that's an expectation of this person. LOL! Everyone will probably running on hydrogen 30 years before that. I've seen guys who virtually never change oil get 100k out cars. Of course they're driving a lot more than 4000 miles per year too.

jmo