Hey Big Bash ....remind me never to buy a motor from you!?
I see two main reasons for changing your oil regularly:
1) Peace of mind on track days due to abnormal sustained high revs / high engine stresses / rich fuelling as Mike has alraedy explained.
2) On the other hand, if youre like me and do lots of short journeys every day, the engine is running richer most of the time and the engine/oil never really heats up properly. Fuel and moisture therefore mixes with the oil and does not evaporate fully. This all ends up in the engine oil (as it should if the detergents are working properly) hence why your oil turns darker. This slowly reduces the oils protection. Granted, synthetic oils are better but not completely infallible.
On the other hand if you are doing around 50 miles non-stop trips on the motorway at lean burning cruising speeds with everything running at correct temperatures, the engine oil is likely to last much longer. I reckon this is probably deemed an "average driving" pattern these days with so many people commuting reasonable distances to work and why oil and car manufacturers are increasing the service intervals. Also under these conditions ECUs and fuel injection systems are providing very accurate metering, reducing the deposits in the oil.
Anyway, here endith my thoughts on the matter ...for what theyre worth
