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While synthetics are more resistant to breakdown from heat, I think what oldyeller said makes perfect sense: you only have a given volume of oil in the engine, no matter what kind of oil it is. Assuming that your engine blows the same amount of crud in to the oil every mile you travel, then for a given amount of mileage driven, you are going to have the same amount of crud suspended in the oil, **no matter what kind of oil it is**. And if you go longer between changes with a synthetic, you are going to have that much more crud suspended in the synthetic oil. The better thermal characteristics of the synthetics may make them last longer before they breakdown and start turning into coke or sludge, but I have never seen any data that indicates they can actually carry more crud per unit volume than the additive package in a good dino oil. Our cars are old, and I'd worry about the crud (blow-by, etc) they are putting in the oil before I'd worry about thermal breakdown. Further, I think you can **effectively** improve a dino oil's resistance to heat breakdown by changing it (very) frequently. It will still be less expensive than using synthetic, and it will do a surprising job of cleaning out the engine and keeping it clean if changed very regularly. On par with synthetic.
A good, reputable machine shop once told me that using inexpensive oil and changing it frequently is much, much better for the engine than using expensive oil and leaving it in the car too long.
In my 1991 745 Turbo, I change the oil every 5 weeks, which works out to be about 2150 miles (the speedo/odo is dead, so we have to approximate mileage). It may be more, or may be less. I use Castrol GTX 10W-30, per the owner's manual. New Mann filter too, that way it never gets dirty and clogged. Clogged oil filters bypass and allow unfiltered, dirty, oil to circulate in your engine.
Used to use Redline Synthetic, but that stuff, while very good, is hellishly expensive and hard to get. Changed it every 8,000 miles (speedo/odo worked back then . . .)
While I like the resistance to heat-induced breakdown and some other properties of synthetics, because I have a turbocharged engine, I think you can come close to approximating that by just changing regular oil more frequently and being more careful. If you have a turbo, just make sure you give it 30 seconds to cool off and spin down before you shut the engine off. If you were just flying down the freeway at 90+mph, maybe make it a minute or minute and a half. No big deal. Stay out of all but minor boost, unless really, REALLY needed, etc (easy to do now that I am 53, and not 23 anymore:))
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)
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