i was just thinking about this, if synthetic is for stop and go driving, how would it handle at highway speeds for long periods of time compared to conventional oil? example mobil 1 5w30 synthetic and regular castrol gtx 5w30.
Thats a good question. I think the question is very complicated. You have the base oil and the additives. The additives can wear out as the oil absorbs impurities and oxidizes. The base oil can "shear down" (becomming thinner) or thicken out. My guess(based on nothing) says that under any conditions the synthetic (PAO) outlasts the Mineral oil by the same percentage.
Thats not an easy question.
All kinds of factors would come into play,
the oil, the vehicle, the condition of the vehicle, the conditions its operating under, the ambient temperature, etc, etc.
But given the choice, I would always go with the synthetic, no matter what the brand.
Look at it this way guys. At a steady highway speed, with the engine warmed up, you don't put a lot of stress on your oil (unless your engine revs at 4000rpm or something!) so you really don't need synthetics for this type of driving.
I'd be willing to bet if someone did a trip across the country and back, and one way they used synthetic and the other way conventional, the wear numbers would be virtually the same.
unfortunately many Hondas (given this users screen name) hover around 4000 rpms at highway speeds, so I say a good synthetic is preferrable, at least for over 4000 mile drain intervals. But I would take Castrol GTX to 4000 miles without any worries though.
I believe that for high speed driving the synthetic is better. Your tendency is to run hotter after about 60 mph from the resistence to air where the engine runs harder. Look at some to the Car and Driver (or other) test reports and you will see that with the smaller engines, top speed is not always in the top gear. In the top gear the engine works HARD in a small engine, causing more wear, more need for the extra bit (albeit small) of hp that a synthetic gives you. Also less apt to oxidize