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I don't mean any offense to Chuck, but I would like to offer another viewpoint on oil.
If you change your oil every 3000 miles you do not need synthetic oil.
Even if you DO change your oil every 3,000 miles, "dino" oil still won't be protecting your engine as well as a quality synthetic. Basically, "dino" oil is distilled, and synthetic is built up to be exactly what is needed. "dino" oil has lots of unwanted components, and needs a more severe additive package just to perform to spec. A quality synthetic oil doesn't really need any additives to perform to spec, but there are often additives in there anyway to perform functions that oil alone cannot do (such as maybe a detergent).
The only other reason to use synthetic is if you are going to be in a situation where the temperature is very cold or very hot.
Actually, another good reason is that your engine, transmission, and transfer case will run cooler with synthetic oil/ATF. With so many people having problems with the Hummer drive train, I would see that synthetic is a smart choice.
Some engines require synthetic because of the advanced nature of the engine. This is especially true of jet engines. Amsoil, the first synthetic for automotive applications, was designed by an Air Force Lt. Col. based on his experience with jet fighters.
If you run with a special dual filtration system you can run the synthetic oil for 10,000 miles.
Your numbers are a little short, at least for Amsoil.
If you use both the Amsoil oil filter and engine oil, you can run for 25,000 miles or 12 months, whichever comes first. And this is without any testing during the interim (you do need to change the oil filter at 6 months or 12,500 miles - sometimes I just change the oil, too).
The combination of Amsoil engine oil, filter, and a bypass filter extends engine oil drain periods to 100,000+ miles. With periodic scientific testing of the oil quality, engines have lasted 400,000 miles because the bypass filter cleans down to .1 micron, which is as clean as a CD manufacturing plant.
As you can probably detect, I happen to believe that the choice of engine oil is critical. I've been using Amsoil for over 8 years. Some people probably don't see this particular maintenance aspect as being quite as important as I see it. No problem, it's your choice. If anyone else happens to share my concern for prolonging engine life, then I hope my comments above were helpful. I hope this doesn't sound more like an Amsoil commercial than simply a Hummer owner who wants to take care of his vehicle so it will last a long time...
I considered using this set up for my gasoline engines. We change our gasoline engine oil every 3,000 to 3,500 miles (except in the Maserati, where it is changed at 2,500 since it has 5,000 mile service intervals, and I feel that is too long for oil, so I just split it in half).
Looking at the cost, it just is not worth going to Amsoil. I would NEVER consider going to 100,000 miles, possibly I might go to 15,000 (two service intervals), so it is just TOO expensive.
Even if I went to 100,000 miles, the filters are still expensive, and you lose some oil changing filters, so it is still more expensive than 3,500 mile oil and filter change intervals. Sure, Amsoil may be better than conventional oil when new, but all you need is one overheating event, and even it is toast (about 250 degrees Fahrenheit).
Since I get well over 200,000 miles, and even 300,000 miles out of every engine anyway, what is to be gained by the extra expense?
In a diesel the situation is much worse. First, diesels really dirty the oil. If you change your oil, and just run your engine until it gets warm, the oil will LOOK dirty. Sure you can filter it, but you can re-sterilize needles and use them over. I have no desire for either.
Diesels have a second, more important problem. The blow by gasses including the sulfur content in the fuel, create sulfuric acid in the oil. In large stationary engines, owners wish to avoid unnecessary oil changes. Oil quantities are measured in gallons rather than quarts, and oil changes are very expensive. In order to extend intervals the oil is often tested for H2SO4. Oil changes are the only practical way to remove the acid. I do not want H2SO4 eating MY engine.
Manufacturers are moving toward a sealed system. BMW now has automatic transmissions with no dip stick. We are heading that way for coolant (antifreeze & water) and I'll bet oil is on the horizon. However, until GM recommends it, I will stick with frequent oil and filter changes. I will bet closed systems occur first with gasoline engines, and possibly never for diesels.
My numbers for the amount of miles driven using synthetic in your engine and a good filter come from Charlie Piper who uses this setup on his 94 diesel. He explained to me that he sent the oil out for testing to determine the duration he uses between changes. I still contend that if you change your engine oil and filter every 3000 miles you don't need synthetic unless you are operating under very extreme conditions. The additives and lubricity in "dino" oil have no problems lasting for 3000 or more miles.
I'm sure some advanced engines require special oils. The GM Diesels are not what I consider advanced engines.
BTW synthetic oil does have additives. Synthetic oil starts with a synthetic base. Read the paper on synthetics I have posted on this web page. This issue came up a couple of years ago and I did a little research on it.
I agree in using synthetics in the axles, geared hubs and maybe in the trans and transfer cases. The transfer case issue is a whole other issue though.
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