15,000 motor oil, synthetic oil i guess you mean. i use it, "fulll synthetic". keep in mind that though the chemical structure is somewhat different than non-synthetic it still gets contaminated from piston ring blow by but is not effected as much as regular motor oil as far as i have reasurched.
then there is 100% real synthetic which is very expensive and relitivly hard to find and still gets dirty.
so what does one do ? to stay within warrenty requirments change motor as per your owners manuel.
i do beleive "full synthetic" is better than std. motor oil (reduced engine wear) from what i have read.
the cost of 100% synthetic makes it out of the question for me.
so for me i use full synthetic in both vehicles and change my oil at about 4500 to 5000k
note "full synthetic" is not 100% synthetic as per a result of a law suit. also i use "castrol syntec" full synthetic " labled not for sale outside the americas due to the lableing not being legal anywhere else i beleive
I gathered from a laboratory analyst working in an oil company that the synthetic component (oil basestock) and additives (detergents and dispersants) used for synthetic engine oils make these oils more resistant to oxidation and do not decompose as readily as mineral based oils. This is the reason why the drain intervals are longer.
However, in my opinion, one must not overlook the API rating of some mineral-based oils that even have higher API ratings than some synthetic oils. Whereas synthetic oils will allow longer drain intervals, an oil with a higher API rating mea! ns that the oil will have higher dispersancy and anti-wear protection. I believe that one can get better engine protection by using an engine oil with the highest API rating available and changing oil as frequently as one can afford to (in terms of time and money) or at an interval not to exceed Kia’s recommended intervals. I know of a car owner who did this and it did keep the internals of his engine very clean indeed. The metal surfaces remained bright and shiny.
Moreover, one must not simply look at the oil to see if it is already “dirty” and use this as a basis for an oil change. Dark colored oil could simply mean that the oil is doing a good job of dispersing the contaminants in the oil and keeping it from depositing or adhering to the internal surfaces of the engine. This is especially true for gasoline engines that employ anti-air pollutio! n devices such as the Exhaust Gas Recirculator, which tend to “dirty” the oil faster.
Also, one must not lose sight of the role of a good oil filter. Generally speaking, the finer the pores of the filter element, the better will it’s ability to trap solid impurities in the oil. However, this will also cause the filter to clog faster. Therefore, the best filter would be one that has a small micron rating but a large effective surface area. I would like to see an independent laboratory do a side-by-side test of OEM and after-market filters.