This is a direct quote from Castrol, dated 2-7-05:
"Using SYNTEC does not directly translate into extended oil change intervals for any vehicle that uses it. Some vehicle manufacturers have oil drain intervals up to 10,000 miles with their vehicles. This is only when the vehicle is equipped with an onboard diagnostics system and oil condition monitors that evaluate a multitude of factors in determining the oil drain interval.
Castrol leans toward the conservative side, and recommends following the guidelines of the manufacturer which can be found in your vehicle's manual for the specific type of driving you do.
This is important because combustion byproducts and other contaminants are still going to build up in any type or brand of motor oil. A number of factors including the engine design, type of driving, weather conditions, oil filter used, etc. determine how much buildup is created.
Today’s full-flow filters effectively remove abrasives from the entire output of the oil pump under normal conditions. However, the oil filter does not remove by-products of combustion such as acid, soot, abrasives, water and unburned fuel contaminants. All of these substances can have damaging effects on both the performance and life of your engine. Therefore, it is very important to change the oil and filter regularly.
We recommend for ultimate protection that you change your motor oil every 3,000 miles (or 3 months).
We have tested our oil under considerably higher oil change intervals and it has performed very well. However, with the number of unknowns as mentioned above, we must err on the side of caution and reiterate that the manufacturers' recommendation be followed."