I’ll redirect you to my quote taken from the manufacturer’s manual above.
Balthisar,
Thank you immensely for the clarification. I managed to misread that verbiage, even though I HAVE a 2002 Mercury with what I think is that exact language in it.
I guess the distinction is that I needed to take the word “primarily” literally. Before, I had not.
Maybe out in Mexico you might classify yourself as a “suburburban” driver but in Toronto (a city with 40% of Ontario’s population) many of us actually drive in the “severe” range of driving.
I classify myself as a “normal” schedule driver (long high speed trips 100 km per day) but the vast majority of Toronto drivers probably are classed in the severe range. I know I did when I worked in the city (a 4 mile drive one way). And daily “walking speed” traffic is a fact of life. I guess I should have clarified my position as “99% of city drivers are severe condition drivers”.
I cringe over the fact that many here think switching to synthetic magically allows them to double their oil change intervals. Even at 6500 km (under Toyota’s recommended interval) my oil comes out pretty dark. Doubling that sounds insane.
Even the best oils get dirty. Even if the oil doesn’t break down, you’re basically sanding down your engine parts when you drive with jet black oil (synthetic or not).
My understainding is that black oil proves the oil is working, holding containimants in suspention.
Well, yes, black oil means that dirt has collected in the oil (including gasoline residue and water). Now that it’s in the oil, you need to change it. A regular oil filter cannot/does not filter well enough to rely on (it’s designed to remove large particles).
Keeping jet black oil in your engine is basically sanding it down. You can even feel the grit if you rub your hands in it (I used to work as one of those oil change guys). Sure, your engine probably will last longer than your transmission (the most abused component in a car IMO) but it doesn’t mean you should.
Another reason to buy new and not trust a used car 
Oops… I should point out that I’m from the Detroit suburbs and that my station in Mexico is temporary (although the UAW is slowly causing this to be more permanent).
I think sitting on QEW at rush hour is almost enough to qualify for severe usage – done that more times than I like to recount! 
I think you get less severe driving on the Toronto surface streets (done that enough, too, but I don’t mind recounting it). Toronto’s a great car city.
The tendancy for a lot of people is to think that their driving is more severe – it’s been beat into our heads by the fast-lube places, dealers, and others who have a lot to gain by getting us to change our oil more often. Really, the automakers would also want to promote too-often lubing: they want (1) fame for quality and long lasting engines, too, and (2) support their dealer networks by promoting dealer services such as frequent oil changes. The fact that the manufactures have a lot to gain but don’t harp on about it says a lot.
We really should take a poll of people with GM oil minders, and see what ranges they get on their oil changes. My last GM product was a V6 Bonneville, and I always got intervals between 5,500 miles and 8,500 miles. And that was heavy stop and go, low-speed use, that everyone’s going on about here. I wish my Ford’s had all the bells and whistles, but they don’t, so I can’t help in a current poll.
I think it has been posted in this thread but - You can’t tell anything about oil by looking at it. The only way to determine if oil is still ‘good’ is to send it to a lab and have them test it.
These guys are testing various synthetic oils and sending samples to a lab every 1000 miles. So far Amsoil went 1 year and 14,000 miles while remaining in spec (except the viscosity changed near the end). Mobil 1 went 18,000 miles in one year and remained in spec.
Wow, great article. I nominate you for most informative post here so far!
I agree, great research.
I’m still mulling over the numbers but I want to point out that the engine used in this test was new and the guys doing the test were car guys.
The reason I bring that up is because when I worked as an oil change monkey most (something like 85%) of people had no clue about their car’s engine. Most were older model vehicles (+5 years) and had all sorts of problems due to very poor oil maintance (note, not just changes, but top ups etc).
As I said before, engine internals probably will not fail due to fequent/non-fequent oil changes (as the test shows, not changing your oil for 8000+ miles seems to be good for an engine where as most people I’m sure follow the manufacturers 3000/5000 mile intervals without any problems).
The engine in the test is also what I would consider an “above average” engine. In that it’s not a junky I4 in a Neon but a nice LS1. A “regular” engine may actually end up burning oil as per it’s regular operation limits were as my 2zz Toyota engine looses less than a quarter litre per 5000 miles. This can matter in these long-interval changes unless you keep track of it (again, most people don’t know how to check their oil).
I’m surprised at the Amsoil rating changing to a 15w40 weight after the test. Not something I’d want to happen in Canada where 5w is absolutely required.
I guess in the end, if you understand exactly what you’re doing when you exceed the recommended interval, there’s no harm (and maybe some good) to the engine. I just wouldn’t recommend it though to just anybody. My new-ish engine gets the recommended changes only because of the warrenty and the fact that it gets beat up on the track.
Just found this discussion about synthetic oil. One of the posters is an engineer with GM.