You should change your oil as recommended by the manufacturer. You change the oil because it gets dirty and the type of oil you use won't change the fact that it will get dirty.
That's easy synthetic oil. Ill tell you why it sounds better
Yes, you can even mix them together.
Two changes you can make to reduce your carbon footprint are drive your car less and use less utilities in your home. Both of these are simple and efficient ways of using less fossil fuels.
Sludge is usually caused by the presence of water in the oil, and can accumulate with use. Ways to minimise sludge production and accumulation includes performing frequent oil changes, performing mechanized engine flushing, or de-sludging, using modern synthetic oils and always following the manufacturer's engine maintenance routine, particularly if the vehicle is oftern used for short urban runs.
Most manufacturers and synthetic oil companies recommend 7500 maintenence intervals with synthetic oil. However, not all "synthetic" oils are true synthetics so be aware of what type/brand of oil you are using. In addition if you drive mostly highway with very little stop and go, city driving you can easily go 10,000 miles between changes.
Using synthetic materials, are less expensive for the suppliers to buy.
Using wiki.answers.com on a frequent basis.
Yes...Some synthetic motor oils recommend 15,000 miles between oil changes. However they also recommend changing the oil filter at 7500 miles then adding more synthetic oil to replace the oil lost from changing the filter. In summary...drive 7500 miles change filter,add oil, then drive another 7500 miles. Read the recommendation on the oil bottle.
The chemicals that are in them
it feels better
Read your drivers manual. Your vehicle may require a synthetic oil.
Using a high-quality synthetic engine oil with a low viscosity can help reduce friction in a car engine by providing better lubrication between moving parts. It is important to follow the manufacturer's recommendations for oil viscosity and change intervals to optimize engine performance and reduce wear.