[Focus man. Focus.]

Working for Castrol I have learned that NEVER go over the recomended service interval when running synthetic oil on a car covered under a warranty. You can do that after you warranty is gone. You will never convince your service department that your 8000 or 10000km oil change interval wasn't the reason your engine failed if that was ever to happen (not saying that it will but why be sorry). Here are some points taken right from the Castrol web site. I know its normal marketing talk but you can say this about any good synthetic motor oil.

Quote:  SYNTEC provides superior protection against deposits. Corrosive particles such as acid, soot and oxidized fuel fragments can cause costly damage to critical engine parts.

SYNTEC contains a Powerful Additive Package that neutralizes corrosive particles, preventing them from grouping together and forming sludge.  Neutralizes acids in your engine that can cause rust and corrosion on vital engine parts such as cylinders, bearings and hydraulic valve lifters.

SYNTEC meets the world's toughest gasoline engine protection requirements.

SYNTEC provides Stability & Endurance under extreme conditions (heat, load, speed) that can cause conventional oils to break down.

SYNTEC delivers the ultimate performance under high-temperature conditions: unsurpassed protection against volatility burn-off and viscosity increase.

SYNTEC can be used with confidence in every gasoline engine passenger car, new or old, regardless of oil previously used.

SYNTEC is fully compatible with all conventional and synthetic oils.

SYNTEC offers a level of protection that Outperforms All Leading Conventional Oils, passing severe industry torture tests.

I am in no way pushing Castrol oil on anyone but I am showing the bennifits of synthetic's in general. Take the above in any way you wish.