First off, I will try to avoid the "synthetics vs conventional" squabble. Oil still gets dirty or contaminated be it synthetic or conventional. It has been my experience and belief that how often you change is more important than what you change with. In other words, I would take 3K changes of conventional 15w40 to 10K or 15K changes of Mobil 1. If you like the synthetic you were running then it shouldn't hurt anything. Especially if you do alot of sub-freezing cold starts. The oil indicator specified changes should be followed, but if I intended on keeping a car past 100K I might consider more frequent changes than 10K intervals. IF it is a leased car...just follow the computer's schedule. See your owners manual for specific weight recommendations whichever path you choose. Oh, almost equally important in my opinion. Try to take it easy during the first few thousand miles of your car's life. Proper break-in seems to be really have an affect on the longevity of a car. You hear various things but I seem to remember the saying "don't exceed 4000rpm for 4K miles". Also taking it easy until the engine is warmed up seems to prolong its lifespan.