It's quite a heady topic. There is a LOT of information on-line about oil change intervals, so you can do some mouse-research. The big problem is everybody's driving styles, and conditions are different, so the only way to REALLY know is to have your used oil tested. If you plan on keeping you car indefinitely then frequent oil changes certainly won't hurt the engine. IMO, 10K is a little long unless you have really good conditions and tend to put on a lot of highway miles. 7500 is probably OK, and there was someone who posted good results when they had the used oil tested, but there isn't any way to compare his driving conditions with yours. I do it at 5K myself, mostly since I know that is a conservative interval AND I can remember it odometer-wise (5k, 10k, 15k, 20k etc). 5K is pretty conservative -- you likely won't need to change it any more frequently than that [using synthetic oil].I've just purchased my first Smart Car Fortwo and I'm super excited about it! The only question I have is about oil changes. As it uses synthetic oil, do I need to change the oil less frequently?
I'm also using Rotella T6 but I have 1/10th your mileage. I have had excellent results with Rotella in our motorhome which is why we switched.I use Shell Rotella T6 5w40 full synthetic. My car is one of the higher mileage smarts here at almost 110,000 miles..